This term is perhaps used more in horary astrology, rather than in natal astrology.- A planet is combust when it is in conjunction with the Sun and therefore hidden from sight by the light of the Sun. Traditionally this is a serious debility and implies that the planet is weakened or restricted in power. However, if the planet is within 17 minutes of the Sun, it is termed Cazimi - in the heart of the Sun - and considered strengthened by the union.
(Skyscript)
I would like to ask you what is your experience of combustion in natal astrology? I looked into a few charts and I don't see any convincing evidence/points that combustion is actually debilitating in natal astrology. It seems much more like a strength to a planet to be conjunct the Sun.
Examples of people with the benefics in combust conjunction to the Sun:
Sun-Jup conjunction:
Adele, Brooke Shields, Vanessa Williams, Eddy Izzard, John Campbell, Fraz Kafka, Kanye West, Mick Jagger, Gerard, Depardieu, Bob Dylan, Woody Allen, Mike Tyson.
Sun-Ven conjunction:
Leonardo DiCaprio, Oprah Winfrey, Demi Moore, Ashton Kutcher, Jim Carrey, Jimi Hendrix, Robert Downey Jr., Ellen Degeneres, Khloe Kardashian, Pink, Daniel Day-Lewis, Tori Amos.
None of these people seem to have a debilitated benefic by being combust, or in conjunction with the Sun - but rather the opposite. Great successes with Jupiter and Venus.