Narcotic: retired army doctor secures more in Milan (5)
I believe the answer is:
opium
'narcotic' is the definition.
(opium is a kind of narcotic)
'retired army doctor secures more in milan' is the wordplay.
'retired' says the letters should be written in reverse.
'army doctor' becomes 'mo'.
'secures' means one lot of letters goes inside another.
'more in milan' becomes 'piu' ('more' in Italian).
'mo' back-to-front is 'om'.
'om' enclosing 'piu' is 'OPIUM'.
(Other definitions for opium that I've seen before include "powerful drug" , "Narcotic drug" , "Narcotic derived from poppy" , "Number" , "Duller" .)