Ignited high explosive fluid (5)
I believe the answer is:
lithe
'fluid' is the definition.
(I've seen this in another clue)
'ignited high explosive' is the wordplay.
'ignited' becomes 'lit' (synonyms).
'high explosive' becomes 'he'.
'lit'+'he'='LITHE'
(Other definitions for lithe that I've seen before include "Slim and supple" , "Elastic" , "Supple and graceful" , "Don -, opera by Verdi" , "Slim, graceful" .)