Hermits occasionally getting into sex that's explosive (6)
I believe the answer is:
semtex
'that's explosive' is the definition.
(I've seen this before)
'hermits occasionally getting into sex' is the wordplay.
'occasionally' indicates alternate letters.
'getting into' means one lot of letters goes inside another.
The alternating letters of 'hermits' are 'emt'.
'emt' inserted inside 'sex' is 'SEMTEX'.
(Other definitions for semtex that I've seen before include "Explosive material used by terrorists" , "Explosive commonly used by terrorists (6)" , "Plastic explosive used by terrorists" , "Plastic explosive from the Czech Republic" , "Type of plastic explosive" .)