In the main it's a bad drug (13)
I believe the answer is:
antihistamine
'drug' is the definition.
(antihistamine is a kind of drug)
'in the main it's a bad' is the wordplay.
'bad' indicates an anagram.
'in'+'the'+'main'+'its'+'a'='inthemainitsa'
'inthemainitsa' anagrammed gives 'ANTIHISTAMINE'.
(Other definitions for antihistamine that I've seen before include "Medicine used to treat allergies" , "Drug treating allergies" , "Hay fever treatment" , "Allergy drug" .)