Knee-length garment (5)
I believe the answer is:
tunic
'knee-length garment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tunic that I've seen before include "Military uniform" , "Loose-fitting shirt or cloak" , "Clobber" , "Policeman may be in one" , "Partly uniform" .)