After setback, feline and alien join forces (4,2)
I believe the answer is:
team up
'join forces' is the definition.
(thesaurus)
'after setback feline and alien' is the wordplay.
'after setback' says the letters should be written backwards.
'feline' becomes 'puma' (puma is a kind of feline).
'and' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extra terrestrial).
'puma'+'et'='pumaet'
'pumaet' written backwards gives 'TEAM UP'.
(Other definitions for team up that I've seen before include "Unite for a common goal" , "Join" , "Brighton and Hove Albion following promotion?" , "Couple" .)