Doctor finds the French spy (4)
I believe the answer is:
mole
'spy' is the definition.
(I know that mole is a type of spy)
'doctor finds the french' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'finds' means one lot of letters go next to another.
'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" .)