Grace Kelly ultimately supporting lover with tango (6)
I believe the answer is:
beauty
'grace' is the definition.
(thesaurus)
'kelly ultimately supporting lover with tango' is the wordplay.
'ultimately' indicates one should take the final letters.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'lover' becomes 'beau' (beau is a kind of lover).
'with' is a charade indicator (letters next to each other).
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
The last letter of 'kelly' is 'y'.
'beau'+'t'='beaut'
'y' put after 'beaut' is 'BEAUTY'.
(Other definitions for beauty that I've seen before include "Very fine specimen" , "See 40" , "Excellent specimen" , "very attractive lady" , "Good looks" .)