City of London finance assumed to be productive (6)
I believe the answer is:
fecund
'productive' is the definition.
(similar in meaning)
'city of london finance assumed' is the wordplay.
I cannot quite see how this works, but
'city' could be 'ec' (the City of London postcode) and 'ec' is present in the answer.
'finance' could be 'fund' (funding is a kind of financing) and 'fund' is found in the remaining letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'to be' is the link.
(Other definitions for fecund that I've seen before include "Highly productive" , "Fertile or productive" , "Prolific, fertile" , "Highly fertile" , "Producing many offspring or ideas" .)