Become aware of warning (6)
I believe the answer is:
notice
This clue is a double definition.
'become aware' is the first definition.
(I've seen this before)
'warning' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for notice that I've seen before include "Notification" , "Advance information" , "Advance warning of job resignation" , "Observe, become aware of" , "'Take heed of, perceive (6)'" .)