Opposed to that man's name - it involved a drug (13)
I believe the answer is:
antihistamine
'a drug' is the definition.
(antihistamine is a kind of drug)
'opposed to that man's name it involved' is the wordplay.
'opposed to' becomes 'anti' (I've seen this in another clue).
'that man's' becomes 'his' (belonging to a man).
'involved' is an anagram indicator (involved can mean confusing or complex).
'name'+'it'='nameit'
'nameit' anagrammed gives 'tamine'.
'anti'+'his'+'tamine'='ANTIHISTAMINE'
(Other definitions for antihistamine that I've seen before include "Medicine used to treat allergies" , "Allergy drug" , "Hay fever treatment" , "Drug treating allergies" .)