Building toy stage, round (4)
I believe the answer is:
lego
'building toy' is the definition.
(toy bricks)
'stage round' is the wordplay.
'stage' becomes 'leg' (synonyms).
'round' becomes 'o' (round can mean a round object).
'leg'+'o'='LEGO'
(Other definitions for lego that I've seen before include "Toy building blocks" , "Children's building bricks" , "Toy building-bricks" , "Kids' bricks" , "Toy with interlocking blocks" .)