Rotter's love, great mistake (6)
I believe the answer is:
toerag
'rotter's' is the definition.
(both can mean an unpleasant person)
'love great mistake' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'mistake' indicates anagramming the letters.
'o'+'great'='ogreat'
'ogreat' anagrammed gives 'TOERAG'.
(Other definitions for toerag that I've seen before include "despicable sort" , "Contemptible person (colloq.)" , "contemptible sort" , "Despicable person (slang)" , "O, great! (anag) -- a worthless fellow" .)