Most unattractive (7)
I believe the answer is:
ugliest
'most unattractive' is the definition.
('ugly' is associated in meaning with 'unattractive')
This is the entire clue.
(Other definitions for ugliest that I've seen before include "superlatively unattractive" , "Most unattractive in appearance" , "Most deficient in beauty" , "Most repulsive" , "Least pretty" .)