A poet is affected by drugs (7)
I believe the answer is:
opiates
'drugs' is the definition.
(opiate is a kind of drug)
'a poet is affected' is the wordplay.
'affected' indicates an anagram.
'a'+'poet'+'is'='apoetis'
'apoetis' is an anagram of 'OPIATES'.
'by' acts as a link.
(Other definitions for opiates that I've seen before include "Sedative drugs" , "Poppy-derived drugs" , "Is a poet (anag.)" , "Narcotic drugs" , "Drugs that dull the feelings, sedatives" .)