Mercury is frequently combust. It kind of makes sense when you think of all the people in the world who don't seem to think at all, though some argue that Mercury is not as harmed by combustion as other planets. A planet is at the end of a cycle when it's combust - it's travelled back to the Sun, so it's weak from the journey and weak from the burning rays. In practical terms, the Sun overpowers it - whatever the planet might do, it tends to do in the Sun's service only, and sometimes it doesn't do things terribly well at all - the Moon especially. In my expereince in natal work, combustion will give one great gift in the area of life represented by the combust planet, and mess up the rest of the areas signified by the planet, though I haven't had enough charts with a combust planet to really write that as an aphorism - but keep an eye out for it, and see if it bears out in your practise. It may or may not do. A combust planet, especially the first or seventh house ruler, often denies marriage in a woman's chart.
Now - if the Sun signifies you - or the quesited, and Mercuy is the other significator, it usually isn't considered the same as combustion, because that would mean the Sun as significator couldn't conjunct anything.
If the Sun isn't a significator, then yes, it's technically combust within 8.5 degrees (as Dr Farr noted, there have been, and still are, many definitions of combustion, but 8.5 is the general standard). Any planet within 15 degrees of the Sun is considered 'under the beams' and therefore weaker than normal, but still functional Combustion and USB do cross sign boundaries. The further from the Sun, the better the combust planet will operate, as a rule.
The exception is cazimi, which makes a planet very strong, and that's when it's within 17 minutes of the Sun, or in the heart of the Sun. I have not noticed this to have great effect if the planet is in poor dignity - such as Mercury in Pisces or Venus in Virgo - it just won't make up for that much debility, but Jupiter and Venus cazimi in Pisces (the ruler and exaltation ruler, respectively), as we had this year, was a helpful thing in horaries.
Again, if the Sun is a significator and it conjuncts the other significator, then it's considered a normal answer (the two planets come together), and not as combustion.
Check
this article by Abu Ma'shar (Albumasar in Latin) at the Cielo é Terra web site. You might find the first paragraph a bit confusing, so skip it if you like and go to number 178 onwards (it's only a few paragraphs down and they each bear a number) to read about the effects of combustion, cazimi, etc. It's good stuff.
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Heartbreaking but I have to learn not to ask unless I can handle the answer.