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' means to take half the letters.
'finished' with half the letters taken 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" .)