Drug worker, one and his affected inmate (13)
I believe the answer is:
antihistamine
'drug' is the definition.
(antihistamine is a kind of drug)
'worker one and his affected inmate' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'one' becomes 'i' (Roman numeral).
'and' means one lot of letters go next to another.
'affected' is an anagram indicator.
'inmate' with letters rearranged gives 'tamine'.
'ant'+'i'+'his'+'tamine'='ANTIHISTAMINE'
(Other definitions for antihistamine that I've seen before include "Allergy drug" , "Medicine used to treat allergies" , "Drug treating allergies" , "Hay fever treatment" .)