Mince pie to include a drug (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'mince pie to include a' is the wordplay.
'mince' is an anagram indicator.
'include' means one lot of letters goes inside another.
'pie'+'to'='pieto'
'pieto' is an anagram of 'opite'.
'opite' enclosing 'a' is 'OPIATE'.
(Other definitions for opiate that I've seen before include "A poet I become on this drug" , "Type of drug" , "Drug inducing a false sense of contentment" , "Poppy-based drug" , "It induces sleep" .)