Not now that "41" has caught cold (4)
I believe the answer is:
once
'not now' is the definition.
(at one time, but not any more)
'41 has caught cold' is the wordplay.
'4o' becomes 'one' (I can't justify this - if you can you should give a lot more credence to this answer).
'has caught' means one lot of letters goes inside another.
'cold' becomes 'C' (eg on taps).
'one' placed around 'c' is 'ONCE'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for once that I've seen before include "Formerly; as soon as" , "not repeatedly" , "Quick examination" , "Previously" , "At some former time" .)