Thomas and Leo are brawling. It’s horrible (9)
I believe the answer is:
loathsome
'horrible' is the definition.
(thesaurus)
'thomas and leo are brawling' is the wordplay.
'and' says to put letters next to each other (I've seen this in other clues).
'are brawling' indicates an anagram.
'thomas' after 'leo' is 'leothomas'.
'leothomas' is an anagram of 'LOATHSOME'.
'it's' is the link.
(Other definitions for loathsome that I've seen before include "gross" , "Repulsive" , "Disgusting, nauseating" , "Detestable" , "Hateful" .)