Throw light on a building (4)
I believe the answer is:
shed
This could be a double definition.
'throw light' is the first definition.
'shed' can be an answer for 'throw' (shedding is a kind of throwing). I'm not sure about the 'light' bit.
'a building' is the second definition.
(shed is a kind of building)
'on' acts as a link.
Can you help me to learn more?
(Other definitions for shed that I've seen before include "Separate" , "Get rid of -- moult" , "Remove clothes in small hut" , "Cast, wooden building" , "Get rid of - garden building" .)