Boot in sports car working well at first (10)
I believe the answer is:
wellington
'boot' is the definition.
(Wellington boots are a type of boot)
'sports car working well at first' is the wordplay.
'sports car' becomes 'GT' (abbreviation for grand tourer).
'working' becomes 'on' (on can mean working or activated).
'at' becomes 'in' (synonymous in some cases - eg at school, in school).
'first' is a charade indicator (letters next to each other).
'well'+'in'='wellin'
'gt'+'on'='gton'
'gton' after 'wellin' is 'WELLINGTON'.
'in' is the link.
(Other definitions for wellington that I've seen before include "Kind of boot" , "Former PM" , "Bomber" , "beef-dish?" , "way to serve beef tenderloin" .)