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