Fringes of undergown tucked into jerk's outfit (5)
I believe the answer is:
tunic
'outfit' is the definition.
(historical type of garment)
'fringes of undergown tucked into jerk's' is the wordplay.
'fringes of' suggests removing the centre.
'tucked into' is an insertion indicator.
'jerk' becomes 'tic' (jerking movement by the body).
'undergown' with its centre removed is 'un'.
'un' inserted into 'tic' is 'TUNIC'.
(Other definitions for tunic that I've seen before include "Cut in (anag)" , "Smock" , "Close fitting jacket as part of uniform" , "Policeman may be in one" , "Knee-length cloak" .)