Explosive relations stifling mother occasionally (6)
I believe the answer is:
semtex
'explosive' is the definition.
(I know that semtex is a plastic explosive)
'relations stifling mother occasionally' is the wordplay.
'relations' becomes 'sex' (euphemism for sex).
'stifling' indicates putting letters inside.
'occasionally' indicates alternate letters.
The alternate letters of 'mother' are 'mte'.
'sex' enclosing 'mte' is 'SEMTEX'.
(Other definitions for semtex that I've seen before include "Type of plastic explosive" , "Plastic explosive used by terrorists" , "Plastic explosive from the Czech Republic" , "Explosive substance used by terrorists" , "Explosive material used by terrorists" .)