Inspect former atomic explosive device (7)
I believe the answer is:
examine
'inspect' is the definition.
(thesaurus)
'former atomic explosive device' is the wordplay.
'former' becomes 'ex'.
'atomic' becomes 'a' (eg in A-test, atomic test).
'explosive device' becomes 'mine' (mine is a kind of explosive device).
'ex'+'a'+'mine'='EXAMINE'
(Other definitions for examine that I've seen before include "Test, probe" , "Investigate" , "Check" , "Appraise" , "Question, test" .)