In the past I must have caught cold (4)
I believe the answer is:
once
'in the past' is the definition.
(once upon a time)
'i must have caught cold' is the wordplay.
I cannot really see how this works, but
'caught' could be 'c' (cricket abbreviation) and 'c' is found in the answer.
'i' could be 'one' (Roman numeral for one) and 'one' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for once that I've seen before include "in bygone era" , "Long ago" , "Sometime" , "At some former time" , "Singly" .)