Toy block brand (4)
I believe the answer is:
lego
'toy block brand' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lego that I've seen before include "Building bricks for children" , "Toy with interlocking blocks" , "Child's building bricks" , "Child's plastic construction set" , "Children's building bricks" .)