Fruit with exterior skin that's more unsightly (6)
I believe the answer is:
'more unsightly' is the definition.
('ugly' can be similar in meaning to 'unsightly')
'fruit with exterior skin' is the wordplay.
'fruit' becomes 'ugli fruit' (ugli fruit is a kind of fruit).
'with' means one lot of letters go next to another.
'exterior skin' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that's' acts as a link.
(Other definitions for uglier that I've seen before include "More repulsive" , "More frightful" , "More unattractive or repellent to the eye" , "More unpleasant to look at" , "More displeasing to the senses" .)