Hopeless endeavour (4,5)
I believe the answer is:
lost cause
'hopeless endeavour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lost cause that I've seen before include "Locates us in failed or doomed endeavour" , "Hopeless quest" , "you won't influence him" , "Hopeless campaign" , "patronised by Jude?" .)