Investigate when chopper's sent up with explosive device (7)
I believe the answer is:
examine
'investigate' is the definition.
(examining is a kind of investigating)
'chopper's sent up with explosive device' is the wordplay.
'chopper' becomes 'axe' (something which chops).
'sent up' shows that the letters should be reversed in order.
'with' means one lot of letters go next to another.
'explosive device' becomes 'mine' (mine is a kind of explosive device).
'axe' back-to-front is 'exa'.
'exa'+'mine'='EXAMINE'
'when' is the link.
(Other definitions for examine that I've seen before include "Set papers" , "Inspect carefully" , "Scrutinise, subject to test" , "Appraise" , "Investigate" .)