They retard big-headed libertines (6)
I believe the answer is:
brakes
'they retard' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'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 "Vehicle's stopping mechanism" , "Constraints on bakers" , "carriages" , "In a way, stops" , "They slow or hinder progress" .)