Understood criminal's exchanged pounds for politician (8)
I believe the answer is:
implicit
'understood' is the definition.
(similar in meaning)
'criminal's exchanged pounds for politician' is the wordplay.
I cannot really understand how this works, but
'pounds' could be 'l' (abbreviation e.g. Lsd - pounds, shillings, pence) and 'l' is found within the answer.
'politician' could be 'mp' (Member of Parliament) and 'mp' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for implicit that I've seen before include "Inferred" , "Not in so many words" , "meaning is obvious?" , "I clip Tim (anag.)" , "unreserved" .)