Disheartened lurgy turned nasty (4)
I believe the answer is:
ugly
'nasty' is the definition.
(I've seen this in another clue)
'disheartened lurgy turned' is the wordplay.
'disheartened' means to remove the middle letters.
'turned' indicates an anagram.
'lurgy' with its centre taken out is 'lugy'.
'lugy' is an anagram of 'UGLY'.
(Other definitions for ugly that I've seen before include "Visually unpleasant; dangerous (situation)" , "Repulsive to the eye" , "Unsightly, not pretty" , "Not nice-looking" , "Revolting" .)