Without delay pay for catching cold (2,4)
I believe the answer is:
at once
'without delay' is the definition.
(I've seen this before)
'pay for catching cold' is the wordplay.
'pay for' becomes 'at one' (I've seen this before).
'catching' is an insertion indicator.
'cold' becomes 'C' (eg on taps).
'atone' placed around 'c' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "Octane (anag)" , "'Without delay (2,4)'" , "without hesitation" , "Immediately, this instant" , "'Immediately (2,4)'" .)