Indicate agreement with reserve, there's nothing doing (2,4)
I believe the answer is:
no dice
'there's nothing doing' is the definition.
(both can mean a situation is hopeless)
'indicate agreement with reserve' is the wordplay.
'indicate agreement' becomes 'nod' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'reserve' becomes 'ice' (I've seen this before).
'nod'+'ice'='NO DICE'
(Other definitions for no dice that I've seen before include "indicates lack of success" , "a negative response" , "Absolutely not" , "Emphatic refusal" , "with which many games are unplayable" .)