Awful Lothario initially promises to love setter (9)
I believe the answer is:
loathsome
'awful' is the definition.
(thesaurus)
'lothario initially promises to love setter' is the wordplay.
'initially' says to take the initial letters.
'promises' becomes 'oaths' (I've seen this before).
'to love' becomes 'o' (love means zero in tennis).
'setter' becomes 'me' (the setter of the clue).
The first letter of 'lothario' is 'l'.
'l'+'oaths'+'o'+'me'='LOATHSOME'
(Other definitions for loathsome that I've seen before include "Horrid" , "Repulsive" , "gross" , "Detestable" , "15 [ABHORRENT]" .)