Notice opening, head off certain risk (9)
I believe the answer is:
adventure
'risk' is the definition.
(synonyms)
'notice opening head off certain' is the wordplay.
'notice' becomes 'ad' (short for advert).
'opening' becomes 'vent' (vent is a kind of opening).
'head off' means to remove the first letter.
'certain' becomes 'sure' (synonyms).
'sure' with its first letter taken away is 'ure'.
'ad'+'vent'+'ure'='ADVENTURE'
(Other definitions for adventure that I've seen before include "speculation" , "Exciting exploit" , "Dangerous undertaking" , "Escapade" , "risky undertaking" .)