Second child on slide (4)
I believe the answer is:
skid
'slide' is the definition.
(I know that skid is a type of slide)
'second child on' is the wordplay.
'second' becomes 's' (common abbreviation).
'child' becomes 'kid' (synonyms).
'on' is a charade indicator (letters next to each other).
's'+'kid'='SKID'
(Other definitions for skid that I've seen before include "Of vehicle, slide as on ice" , "Runn" , "Slip out of control" , "Uncontrolled slide" , "Runner" .)