Thrown off a garden building (4)
I believe the answer is:
shed
This clue is a double definition.
'thrown off' is the first definition.
('shed' can be a synonym of 'throw off')
'a garden building' is the second definition.
(I've seen this before)
(Other definitions for shed that I've seen before include "Removed in garden hut" , "Small hut - lose it" , "shelter" , "storage building" , "building away from home" .)