Chemical substance spy used against sappers? (7)
I believe the answer is:
reagent
'chemical substance' is the definition.
(I've seen this before)
'spy used against sappers?' is the wordplay.
'spy' becomes 'agent' (I've seen this before).
'used against' is a charade indicator (letters next to each other).
'sappers?' becomes 're' (Royal Engineers).
'agent' after 're' is 'REAGENT'.
(Other definitions for reagent that I've seen before include "Substance with characteristic reactions, used as a chemical test" , "Chemical with known properties" , "will cause some reaction" , "Reacting substance" , "Chemical substance used in tests" .)