Ill-natured agitated guy, about fifty (4)
I believe the answer is:
ugly
'ill-natured' is the definition.
('ugly' can be similar in meaning to 'ill-natured')
'agitated guy about fifty' is the wordplay.
'agitated' is an anagram indicator.
'about' indicates putting letters inside.
'fifty' becomes 'l' (roman numeral).
'guy' with letters rearranged gives 'ugy'.
'ugy' enclosing 'l' is 'UGLY'.
(Other definitions for ugly that I've seen before include "Evil-looking" , "Offensive to one's sight or other senses" , "'Unattractive, unsightly (4)'" , "Visually unpleasant; dangerous (situation)" , "Frightful" .)