Country work unit returned nothing (7)
I believe the answer is:
grenada
'country' is the definition.
(Grenada is an example)
'work unit returned nothing' is the wordplay.
'work unit' becomes 'erg' (erg is a kind of work unit).
'returned' says the letters should be written in reverse.
'nothing' becomes 'nada' ('nada' can be a synonym of 'nothing').
'erg' written backwards gives 'gre'.
'gre'+'nada'='GRENADA'
(Other definitions for grenada that I've seen before include "Caribbean country, capital St George's" , "Commonwealth country" , "Island state in the West Indies" , "commonwealth state" , "W, Indies island" .)