Individual catching cold on one occasion (4)
I believe the answer is:
once
'on one occasion' is the definition.
('once' can be a synonym of 'on one occasion')
'individual catching cold' is the wordplay.
'individual' becomes 'one' (**).
'catching' is an insertion indicator.
'cold' becomes 'c' (eg on taps).
'one' placed around 'c' is 'ONCE'.
(Other definitions for once that I've seen before include "One-time musical film and song" , "In former times" , "Just as a single occurrence" , "Some time back" , "Previously" .)