Fruitful supply across the City (6)
I believe the answer is:
fecund
'fruitful' is the definition.
(thesaurus)
'supply across the city' is the wordplay.
'supply' becomes 'fund' (I've seen this before**).
'across' means one lot of letters goes inside another.
'the city' becomes 'ec' (the City of London postcode).
'fund' going around 'ec' is 'FECUND'.
(Other definitions for fecund that I've seen before include "Prolific, fertile" , "Highly productive" , "Fertile or productive" , "Highly fertile" , "Producing many offspring or ideas" .)