Narcotic: retired army doctor secures more in Milan (5)
I believe the answer is:
Here is my best explanation:
'narcotic' is the definition.
(opium is a kind of narcotic)
'retired army doctor secures more in milan' is the wordplay.
'retired' is a reversal indicator.
'army doctor' becomes 'mo'.
'secures' means one lot of letters goes inside another.
'more in milan' becomes 'piu' ('more' in Italian).
'mo' in reverse letter order is 'om'.
'om' placed around 'piu' is 'OPIUM'.
(Other definitions for opium that I've seen before include "A narcotic", "Narcotic derived from poppies", "Poppy-based drug", "derivative", "wars with China", "Addictive drug made from poppies", "Poppy narcotic", "powerful drug", "Drug from poppy" and "narcotic drug".)