Messed up final -- galling (7)
I believe the answer is:
painful
'galling' is the definition.
(thesaurus)
'messed up final' is the wordplay.
'messed' is an anagram indicator (I've seen this in other clues).
'up'+'final'='upfinal'
'upfinal' with letters rearranged gives 'PAINFUL'.
(Other definitions for painful that I've seen before include "Distressing or hurtful" , "Hurting" , "Inflamed" , "Causing hurt, physical or mental" , "Sore" .)