Dined in cosy place, most elegant (7)
I believe the answer is:
neatest
'most elegant' is the definition.
('neat' can be similar in meaning to 'elegant')
'dined in cosy place' is the wordplay.
'dined' becomes 'ate'.
'in' indicates putting letters inside.
'cosy place' becomes 'nest' (I've seen this before).
'ate' placed inside 'nest' is 'NEATEST'.
(Other definitions for neatest that I've seen before include "best-looking desk, say?" , "Most orderly" , "Seat ten (anag)" , "Most tidy" , "Most smart" .)