Cast off clothing (9)
'cast off clothing' is the definition.
'knitwear' can be an answer for 'clothing' (knitwear is a kind of clothing). I am not sure about the 'cast off' bit.
'cast off clothing' is the wordplay.
'cast off' becomes 'knit' (I can't explain this - if you can you should give a lot more credence to this answer).
'clothing' becomes 'wear' ('wear' can be a synonym of 'clothing').
'knit'+'wear'='KNITWEAR'
(Other definitions for knitwear that I've seen before include "Woollens" , "Jersey etc ..." , "'Woolly jumpers, socks etc (8)'" , "stockings, perhaps" , "Woollen clothing" .)
(Other definitions for undressed that I've seen before include "Naked" , "Stripped" , "Exposed" , "Lacking kit" , "Removed clothing" .)