Building block company (4)
I believe the answer is:
lego
'building block company' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lego that I've seen before include "Toy building bricks" , "Building bricks for children" , "constructed without care" , "Toy building-bricks" , "Toy with interlocking blocks" .)