Former ace catching cold (4)
I believe the answer is:
once
'former' is the definition.
(synonyms)
'ace catching cold' is the wordplay.
'ace' becomes 'one' (synonyms).
'catching' means one lot of letters goes inside another.
'cold' becomes 'C' (eg on taps).
'one' going around 'c' is 'ONCE'.
(Other definitions for once that I've seen before include "A single time" , "Cone (anag.)" , "Just as a single occurrence" , "Long ago" , "Quick examination" .)