Garment hack admits wearing back-to-front (5)
I believe the answer is:
tunic
'garment' is the definition.
(historical type of garment)
'hack admits wearing back-to-front' is the wordplay.
'hack' becomes 'cut'.
'admits' is an insertion indicator (to admit is to allow in).
'wearing' becomes 'in' (you are 'in' the clothes you're wearing).
'back-to-front' is a reversal indicator.
'cut' going around 'in' is 'cinut'.
'cinut' backwards is 'TUNIC'.
(Other definitions for tunic that I've seen before include "Belted coat" , "Loose sleeveless garment" , "Dress for soldier" , "Short uniform coat" , "Jacket of some soldiers" .)