Slip, small, on child (4)
I believe the answer is:
skid
'slip' is the definition.
('skid' can be a synonym of 'slip')
'small on child' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'on' says to put letters next to each other.
'child' becomes 'kid' ('kid' can be a synonym of 'child'**).
's'+'kid'='SKID'
(Other definitions for skid that I've seen before include "Slide (on slippery ground)" , "Lose control of a car" , "Slide uncontrollably" , "Lose lateral control" , "go out of control" .)