Construction set on offer, originally (4)
I believe the answer is:
lego
'construction set' is the definition.
(I know that Lego is a type of construction set)
'on offer originally' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'originally' indicates taking the first letters.
The initial letter of 'offer' is 'o'.
'leg'+'o'='LEGO'
(Other definitions for lego that I've seen before include "Children's building bricks" , "Small bricks" , "Child's plastic construction set" , "Toy building bricks" , "construction kit" .)