Supplement when cold is caught becoming a habit (9)
I believe the answer is:
addiction
'a habit' is the definition.
(thesaurus)
'supplement when cold is caught' is the wordplay.
I cannot really understand how this works, but
'supplement' could be 'add' (I've seen this before) and 'add' is located in the answer.
'cold' could be 'c' (eg on taps) and 'c' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for addiction that I've seen before include "Obsession" , "I did act on (anag.)" , "hard to stop" , "Something to kick" , "Bug one's caught" .)