Ill-natured help catching cold (4)
I believe the answer is:
acid
'ill-natured' is the definition.
(I've seen this in another clue)
'help catching cold' is the wordplay.
'help' becomes 'aid' (synonyms).
'catching' is an insertion indicator.
'cold' becomes 'C' (eg on taps).
'aid' placed around 'c' is 'ACID'.
(Other definitions for acid that I've seen before include "Tart - keen - sort of house party" , "So-called drug" , "Substance with pH of less than 7" , "Sour (substance)" , "Morose" .)