City given no backing in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(I've seen this before)
'city given no backing' is the wordplay.
'city' becomes 'ec' (the City of London postcode).
'given' says to put letters next to each other.
'backing' shows that the letters should be reversed in order.
'ec'+'no'='ecno'
'ecno' back-to-front is 'ONCE'.
(Other definitions for once that I've seen before include "In days past" , "Previously" , "not repeatedly" , "At a previous time" , "In times past" .)