Unexpected reactions to drug (9)
I believe the answer is:
narcotise
'drug' is the definition.
(I've seen this before)
'unexpected reactions' is the wordplay.
'unexpected' indicates an anagram.
'reactions' anagrammed gives 'NARCOTISE'.
'to' acts as a link.
(Other definitions for narcotise that I've seen before include "Numb" , "put under influence of drugs" , "administer drug" .)