Awful vote against boxing disheartened sport (5)
I believe the answer is:
nasty
'awful' is the definition.
(I know that awful can be written as nasty)
'vote against boxing disheartened sport' is the wordplay.
'vote against' becomes 'nay' (I've seen this in another clue).
'boxing' indicates putting letters inside ('box' can mean contain, as in to put in a box).
'disheartened' means to remove the middle letters (the middle or heart is removed).
'sport' with its centre removed is 'st'.
'nay' enclosing 'st' is 'NASTY'.
(Other definitions for nasty that I've seen before include "Disagreeable" , "Mean" , "Spiteful and unkind" , "Awful" , "Filthy" .)