Set off ambush reversed in action (8)
I believe the answer is:
departed
'set off' is the definition.
(I've seen this before)
'ambush reversed in action' is the wordplay.
'ambush' becomes 'trap' (synonyms).
'reversed' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'trap' in reverse letter order is 'part'.
'part' placed inside 'deed' is 'DEPARTED'.
(Other definitions for departed that I've seen before include "completely breathless" , "Quit" , "Gone, maybe dead" , "Left, went away" , "Dead; set off" .)