They stop big-headed libertines (6)
I believe the answer is:
'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).
Can you help me to learn more?
(Other definitions for brakes that I've seen before include "They slow or hinder progress", "Slowing or stopping devices", "Slows down", "They work to slow a vehicle", "Vehicle's stopping mechanism".)