Pay the bill (4,2,3,3)
I believe the answer is:
pick up the tab
'pay the bill' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pick up the tab that I've seen before include "Pay for something" , "Pay everyone's bills" , "Pay the whole bill" , "Foot the bill" , "Assume the expense" .)