Employers backing unions' resurgence to some extent (5)
I believe the answer is:
users
'employers' is the definition.
(I've seen this before)
'backing unions' resurgence to some extent' is the wordplay.
'backing' shows that the letters should be reversed in order.
'to some extent' indicates the answer is hidden within the clue.
'sresu' is hidden in the letters of 'unions resurgence'.
'sresu' reversed gives 'USERS'.
(Other definitions for users that I've seen before include "People with a drug habit" , "Drug takers" , "Ruses for employers" , "Customers" , "Drug addicts" .)