Keen helper catching cold (4)
I believe the answer is:
acid
'keen' is the definition.
(I've seen this before)
'helper catching cold' is the wordplay.
'helper' becomes 'aid' (something which helps).
'catching' means one lot of letters goes inside another.
'cold' becomes 'c' (eg on taps).
'aid' enclosing 'c' is 'ACID'.
(Other definitions for acid that I've seen before include "Tart - keen - sort of house party" , "Chemical compound" , "Sharp - tart - kind of house party" , "Sour - type of rain or drop" , "something sharp" .)