Cynthia, terribly good sport (8)
I believe the answer is:
yachting
'sport' is the definition.
(I've seen this before)
'cynthia terribly good' is the wordplay.
'terribly' indicates an anagram.
'good' becomes 'g' (abbreviation).
'cynthia' with letters rearranged gives 'yachtin'.
'yachtin'+'g'='YACHTING'
(Other definitions for yachting that I've seen before include "Sailing for pleasure" , "Itchy nag (anag.)" , "Boating sport" , "Boating on pleasure craft" , "Sailing sport" .)