Cut layer, about a yard — how much do you want? (3,4)
I believe the answer is:
say when
'how much do you want?' is the definition.
(phrase used when pouring a drink)
'cut layer about a yard' is the wordplay.
'cut' becomes 'saw' (sawing is a kind of cutting).
'layer' becomes 'hen' (I've seen this in another clue).
'about' is an insertion indicator.
'a yard' becomes 'y' (abbreviation).
'saw'+'hen'='sawhen'
'sawhen' placed around 'y' is 'SAY WHEN'.
(Other definitions for say when that I've seen before include "Words from the pourer?" , "Tell me when to stop pouring" , "Request to be told to stop pouring drink" , "Pourer's words" .)