They stop big-headed libertines (6)
I believe the answer is:
brakes
'they stop' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'big-headed libertines' is the wordplay.
'big-headed' becomes 'b' (this could be a standard abbreviation I've not previously seen).
'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 "They work to slow a vehicle" , "Slows down" , "Constraints on bakers" , "In a way, stops" , "Thickets of brush or briars" .)