City invested in finance that's productive (6)
I believe the answer is:
fecund
'productive' is the definition.
(similar in meaning)
'city invested in finance' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'invested in' means one lot of letters goes inside another.
'finance' becomes 'fund' (funding is a kind of financing).
'ec' inserted inside 'fund' is 'FECUND'.
'that's' acts as a link.
(Other definitions for fecund that I've seen before include "Highly productive" , "Producing many offspring or ideas" , "Fertile or productive" , "Highly fertile" , "Prolific, fertile" .)