Fruit, cherry in the core is more foul (6)
I believe the answer is:
uglier
'more foul' is the definition.
Both the answer and definition are comparative adjectives. Maybe there's a link between them I don't understand?
'fruit cherry in the core' is the wordplay.
'fruit' becomes 'ugli fruit' (ugli is a kind of fruit).
'cherry in the core' becomes 'er' (I can't justify this - if you can you should believe this answer much more).
'ugli'+'er'='UGLIER'
'is' is the link.
Can you help me to learn more?
(Other definitions for uglier that I've seen before include "with worse features" , "Less beautiful" , "More hideous" , "More displeasing to the senses" , "More unattractive or repellent to the eye" .)