Editor's written about a drug (7)
I believe the answer is:
steroid
'a drug' is the definition.
(type of drug)
'editor's written about' is the wordplay.
'written about' is an anagram indicator (letters moved about before being rewritten).
'editors' with letters rearranged gives 'STEROID'.
(Other definitions for steroid that I've seen before include "muscle-building compound" , "Type of drug, maybe anabolic" , "Compond such as many hormones and vitamins" , "Type of hormone" , "Performance-enhancer illegal for athletes" .)