City backed in company's first-class return (7)
I believe the answer is:
nicosia
'city' is the definition.
(Nicosia is an example)
'backed in company's first-class return' is the wordplay.
'backed' says the letters should be written in reverse.
'company's' becomes 'co's' (abbreviation).
'first class' becomes 'ai' (A1 - top quality).
'return' is a reversal indicator.
'in' in reverse letter order is 'ni'.
'ai' backwards is 'ia'.
'ni'+'cos'+'ia'='NICOSIA'
(Other definitions for nicosia that I've seen before include "It's capital in Cyprus (7)" , "Casino I find in Cyprus - that's capital" , "Mediterranean city" , "Divided Cypriot capital" , "Largest city on Cyprus" .)