Backs finish before Cockney nags (8)
I believe the answer is:
endorses
'backs' is the definition.
('endorse' can be a synonym of 'back')
'finish before cockney nags' is the wordplay.
'finish' becomes 'end' (synonyms).
'before' is a charade indicator (letters next to each other).
'cockney nags' becomes ''orses' ('horses' said with the 'H' dropped - a cockney accent.).
'end'+'orses'='ENDORSES'
(Other definitions for endorses that I've seen before include "'Supports, backs (8)'" , "Gives support or approval to" , "One dress (anag.)" , "Approves, supports" , "Countersigns, approves" .)