It's not the trainer who's the b..... scoundrel! (6)
I believe the answer is:
brogue
'it's not the trainer who's' is the definition.
I can't tell whether this definition defines the answer.
'b scoundrel' is the wordplay.
'scoundrel' becomes 'rogue' (rogue is a kind of scoundrel).
'b'+'rogue'='BROGUE'
'the' is the link.
Can you help me to learn more?
(Other definitions for brogue that I've seen before include "Oxford" , "Kind of shoe" , "Style of shoe - style of accent" , "afoot?" , "Strong outdoor shoe" .)