Slide sideways, initially striking young child (4)
I believe the answer is:
skid
'slide sideways' is the definition.
(I've seen this in another clue)
'initially striking young child' is the wordplay.
'initially' suggests taking the first letters.
'young child' becomes 'kid' (kid can mean a young child).
The initial letter of 'striking' is 's'.
's'+'kid'='SKID'
(Other definitions for skid that I've seen before include "lose control momentarily" , "Slip, slide" , "Slide, eg, after braking" , "Swerve out of control" , "Runner" .)