Measure of liquid has dropped on cheek (4)
I believe the answer is:
gall
'cheek' is the definition.
(both can mean outrageous behaviour)
'measure of liquid has dropped on' is the wordplay.
'measure of liquid' becomes 'gallon'.
'has dropped' indicates named letters should be taken away.
'gallon' with 'on' taken away is 'GALL'.
(Other definitions for gall that I've seen before include "Bitterness of feeling or bodily bladder" , "Effrontery" , "Sheer impudence from bladder?" , "Liver's secretion" , "Irritate - presumption" .)