Pleasant to hold back party? Not a success (2,4)
I believe the answer is:
no dice
'not a success' is the definition.
(used to show a plan has no chance of succeeding)
'pleasant to hold back party?' is the wordplay.
'pleasant' becomes 'nice' (associated in meaning).
'to hold' indicates putting letters inside.
'back' says the letters should be written backwards.
'party?' becomes 'do' (do is a kind of party).
'do' backwards is 'od'.
'nice' going around 'od' is 'NO DICE'.
(Other definitions for no dice that I've seen before include "I refuse to play!" , "Shan't play!" , "Not a chance of co-operation" , "Said to indicate failure" , "Certainly not" .)