Flame once applied to an explosive device in test (7)
I believe the answer is:
examine
'test' is the definition.
(I know that test can be written as examine)
'flame once applied to an explosive device' is the wordplay.
'flame once' becomes 'ex' (an ex is an old flame).
'applied to' means one lot of letters go next to another.
'an' becomes 'a'.
'explosive device' becomes 'mine' (mine is a kind of explosive device).
'ex'+'a'+'mine'='EXAMINE'
'in' is the link.
(Other definitions for examine that I've seen before include "Inspect, peruse closely" , "Inspect , test" , "Question (a witness)" , "Subject to inspection" , "Analyse" .)