What remains after fool picks up high explosive (5)
I believe the answer is:
ashes
'what remains' is the definition.
(remains of a fire)
'fool picks up high explosive' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'picks up' is an insertion indicator.
'high explosive' becomes 'he'.
'ass' enclosing 'he' is 'ASHES'.
'after' acts as a link.
(Other definitions for ashes that I've seen before include "Test-match trophy" , "Eng/Aus test series" , "Residue from fire" , "Cricket honour in the fire" , "Cricketing trophy - remains" .)