Doctor finds the French spy (4)
I believe the answer is:
mole
'spy' is the definition.
(mole is a kind of spy)
'doctor finds the french' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'finds' is a charade indicator (letters next to each other).
'the french' becomes 'le' ('the' in French).
'mo'+'le'='MOLE'
(Other definitions for mole that I've seen before include "Spot - burrowing mammal" , "Infiltrator" , "Spy; burrower" , "Adrian ---, fancies Pandora; lurking spy" , "spy a > breakwater" .)