Fruit that's rotten coming out of one's mouth? (4)
I believe the answer is:
ugli
'fruit' is the definition.
(ugli fruit is a kind of fruit)
'rotten coming out of one's mouth?' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is found within the answer.
The remaining letters 'ug' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for ugli that I've seen before include "Large juicy fruit with thick wrinkled skin" , "(Repulsive-looking?) fruit" , "West Indian citrus fruit" , "Citrus fruit of the West Indies" , "Unattractive-sounding fruit?" .)