Slit in covered up garment (5)
I believe the answer is:
tunic
'garment' is the definition.
(historical type of garment)
'slit in covered up' is the wordplay.
'slit' becomes 'cut' (similar in meaning).
'covered' indicates putting letters inside.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'cut' enclosing 'in' is 'cinut'.
'cinut' backwards is 'TUNIC'.
(Other definitions for tunic that I've seen before include "Schoolgirl's dress" , "piece of clothing" , "Loose fitting light cloak extending to the hip or lower" , "Close-fitting jacket that's part of a uniform" , "Shift" .)