Child put in very old snowmobile (6)
I believe the answer is:
skidoo
'snowmobile' is the definition.
(trademark name for a type of snowmobile)
'child put in very old' is the wordplay.
'child' becomes 'kid' ('kid' can be a synonym of 'child'**).
'put in' is an insertion indicator.
'very' becomes 'so' (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'kid' put inside 'so' is 'skido'.
'skido'+'o'='SKIDOO'
(Other definitions for skidoo that I've seen before include "Snow vehicle with skis at the front" , "winter vehicle" , "Motorised toboggan - get out fast" , "snowmobile" , "Motorised sledge" .)