Support circular toy (4)
I believe the answer is:
lego
'toy' is the definition.
(I know that Lego is a type of toy)
'support circular' is the wordplay.
'support' becomes 'leg' (leg is a kind of support).
'circular' becomes 'o' (I've seen this in other clues).
'leg'+'o'='LEGO'
(Other definitions for lego that I've seen before include "construction kit" , "Toy building bricks" , "Toy building-bricks" , "Toy consisting of interlocking plastic bricks" , "Kids' bricks" .)