Penicillin, eg (10)
I believe the answer is:
antibiotic
'penicillin eg' is the definition.
(penicillin is a kind of antibiotic)
This is all the clue.
(Other definitions for antibiotic that I've seen before include "Infection treatment" , "Medicine destroying micro-organisms" , "Substance that destroys bacteria" , "Medication" , "Remedy" .)