Most elegant dog's seen in court (9)
I believe the answer is:
classiest
'most elegant' is the definition.
(I've seen this before)
'dog's seen in court' is the wordplay.
'dog's' becomes 'lassie's'.
'seen in' means one lot of letters goes inside another.
'court' becomes 'ct' (abbreviation used in road names).
'lassies' going within 'ct' is 'CLASSIEST'.
(Other definitions for classiest that I've seen before include "Most elegant" , "Most sophisticated" .)