Cast off; small building (4)
I believe the answer is:
shed
I believe this clue is a double definition.
'cast off' is the first definition.
(shedding is a kind of casting off)
'small building' is the second definition.
(I've seen this before)
(Other definitions for shed that I've seen before include "Get rid of -- moult" , "storage unit" , "lost" , "place for bikes" , "Simple building; discard" .)