Garden building half finished (4)
I believe the answer is:
shed
'garden building' is the definition.
(I've seen this before)
'half finished' is the wordplay.
'half' indicates taking half.
'finished' cut in half is 'SHED'.
(Other definitions for shed that I've seen before include "Garden feature" , "Throw off; building" , "Discard; small building" , "Drop; tool store" , "Outbuilding for storage" .)