Bricks on top of outbuilding (4)
I believe the answer is:
lego
'bricks' is the definition.
(I have seen 'Small bricks' mean 'lego' so perhaps 'bricks' could also mean 'lego')
'on top of outbuilding' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'top of' suggests taking the first letters.
The initial letter of 'outbuilding' is 'o'.
'leg'+'o'='LEGO'
(Other definitions for lego that I've seen before include "Toy consisting of interlocking plastic bricks" , "Child's building bricks" , "Toy building bricks" , "Toy construction set" , "Toy building blocks" .)