They restrain big-headed libertines (6)
I believe the answer is:
brakes
'they restrain' is the definition.
The definition suggests an adverb but the answer is not.
'big-headed libertines' is the wordplay.
'big-headed' becomes 'b' (this might be a standard abbreviation which I don't know about).
'libertines' becomes 'rakes' (rake is a kind of libertine).
'b'+'rakes'='BRAKES'
Can you help me to learn more?
(Other definitions for brakes that I've seen before include "Vehicle's stopping mechanism" , "Slowing or stopping devices" , "In a way, stops" , "Thickets of brush or briars" , "carriages" .)