Snowmobile soon contracted to carry child (6)
I believe the answer is:
skidoo
'snowmobile' is the definition.
(trademark name for a type of snowmobile)
'soon contracted to carry child' is the wordplay.
'contracted' means to remove the last letter (to contract is to shorten).
'to carry' means one lot of letters goes inside another.
'child' becomes 'kid' (synonyms).
'soon' with its last letter taken off is 'soo'.
'soo' going around 'kid' is 'SKIDOO'.
(Other definitions for skidoo that I've seen before include "Motorised toboggan - get out fast" , "Snow vehicle with skis at the front" , "Motorised sledge" , "winter vehicle" , "snowmobile" .)