Track welcomes non-smoking supporter (7)
I believe the answer is:
sponsor
'supporter' is the definition.
('sponsor' can be a synonym of 'supporter')
'track welcomes non-smoking' is the wordplay.
'track' becomes 'spoor' (spoor is a kind of track).
'welcomes' means one lot of letters goes inside another (in sense of 'invites in').
'non-smoking' becomes 'ns'.
'spoor' going around 'ns' is 'SPONSOR'.
(Other definitions for sponsor that I've seen before include "surety" , "Supporter or champion" , "Patron; promote" , "One paying to support" , "One funding an event" .)