After being single, catching cold (4)
I believe the answer is:
once
'after' is the definition.
(once can mean immediately after)
'single catching cold' is the wordplay.
'single' becomes 'one' (similar in meaning).
'catching' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'one' placed around 'c' is 'ONCE'.
'being' acts as a link.
(Other definitions for once that I've seen before include "In days gone by" , "Sometime" , "On a single (or past) occasion" , "In days past" , "In the old days" .)