The remains of high explosive found in a vessel (5)
I believe the answer is:
ashes
'the remains' is the definition.
(I've seen this before)
'high explosive found in a vessel' is the wordplay.
'high explosive' becomes 'he'.
'found in' is an insertion indicator.
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'a'+'ss'='ass'
'he' placed inside 'ass' is 'ASHES'.
'of' is the link.
(Other definitions for ashes that I've seen before include "Test cricket series" , "Remains of anything burnt" , "` `..... to ....., dust to dust' ' (prayer)" , "Former Wimbledon champion's" , "Eng./Aus. cricket trophy" .)