Stop Liberal in headwear (4)
I believe the answer is:
halt
'stop' is the definition.
(halt can mean to stop)
'liberal in headwear' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'in' means one lot of letters goes inside another.
'headwear' becomes 'hat' (hat is a type of headwear).
'l' inserted within 'hat' is 'HALT'.
(Other definitions for halt that I've seen before include "Stop, small station" , "Stop; minor station" , "Falter - stop" , "Small railway station" , "game" .)