Worsen cute tail on horse in avatar design (9)
I believe the answer is:
aggravate
'worsen' is the definition.
(synonyms)
'cute tail on horse in avatar design' is the wordplay.
'cute tail' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'in' is an insertion indicator.
'design' indicates anagramming the letters.
'avatar' anagrammed gives 'aravat'.
'gg' placed within 'aravat' is 'aggravat'.
'e' put after 'aggravat' is 'AGGRAVATE'.
Can you help me to learn more?
(Other definitions for aggravate that I've seen before include "Pester" , "compound" , "Exasperate or exacerbate" , "Annoy; make worse" , "Make (a problem) worse" .)