After gym, I got into slight brief nightwear (8)
I believe the answer is:
peignoir
'nightwear' is the definition.
'after gym i got into slight brief' is the wordplay.
'after' is a charade indicator (letters next to each other).
'gym' becomes 'pe'.
'got into' is an insertion indicator.
'slight' becomes 'ignore' ('ignore' can be a synonym of 'slight').
'brief' means to remove the last letter.
'ignore' with its final letter removed is 'ignor'.
'i' placed inside 'ignor' is 'ignoir'.
'pe' put next to 'ignoir' is 'PEIGNOIR'.
(Other definitions for peignoir that I've seen before include "Woman's loose full dressing gown" , "habit" , "Woman's light dressing-gown" , "Woman's light dressing gown" , "bathrobe" .)