Immediately compensate for catching cold (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this before)
'compensate for catching cold' is the wordplay.
'compensate for' becomes 'atone' (I am not sure about this - if you are sure you should believe this answer much more).
'catching' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'atone' placed around 'c' is 'AT ONCE'.
Can you help me to learn more?
(Other definitions for at once that I've seen before include "without pause" , "simultaneously" , "This instant - no delay (2,4)" , "right away" , "'Without delay (2,4)'" .)