Joiner in seat wearing garment, a certain style (5,4,3,5)
I believe the answer is:
short back and sides
'a certain style' is the definition.
(hair style)
'joiner in seat wearing garment' is the wordplay.
I cannot really see how this works, but
'joiner' could be 'and' (I've seen this before) and 'and' is located in the answer.
'seat' could be 'backside' (synonyms) and 'backside' is present in the leftover letters.
'garment' could be 'shorts' (shorts is a kind of garment) and 'shorts' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for short back and sides that I've seen before include "Shock reduction" , "Military hairstyle" , "Cut" .)