roey As a general rule, when you ask a question on AB , it would help if you declare the number of letters in the solution and any letters you have (or think you have)!
Do not assume that everyone has a copy of the grid - as it is clear that only one of us had that advantage. The rest of us were solving the clue completely blind.
Chambers defines "penicillin" as "a group of substances that stop ... bacteria, extracted from moulds ..."
Under "antibiotic" it has "a substance produced by micro-organisms which, in dilute solution, has the capacity ..."
The clue which you posed (in the absence of all the possible hints you could have given) fits "penicillin" slightly better than the correct option!
In other words - Please help US to help YOU.