His last child to slide sideways (4)
I believe the answer is:
skid
'slide sideways' is the definition.
(I've seen this before)
'his last child' is the wordplay.
'last' says to take the final letters.
'child' becomes 'kid' (synonyms).
The final letter of 'his' is 's'.
's'+'kid'='SKID'
'to' is the link.
(Other definitions for skid that I've seen before include "Slide (on slippery ground)" , "Of vehicle, slide as on ice" , "Slide at an angle" , "Slip, slide" , "Lose lateral control" .)