Fine skills must underpin good racing vehicles (2-5)
I believe the answer is:
go-karts
'racing vehicles' is the definition.
(I've seen this before)
'fine skills must underpin good' is the wordplay.
'fine' becomes 'ok' (synonyms).
'skills' becomes 'arts' (I've seen this before).
'must underpin' is a charade indicator (letters next to each other) (in a down clue, some letters go beneath others).
'good' becomes 'g' (abbreviation).
'ok'+'arts'='okarts'
'okarts' put after 'g' is 'GO-KARTS'.
(Other definitions for go-karts that I've seen before include "Light vehicles for racing" , "Small motor vehicles" .)