Spirit, primarily rum, in medium strength vermouth (6)
I believe the answer is:
esprit
'spirit' is the definition.
(I know that esprit is a type of spirit)
'primarily rum in medium strength vermouth' is the wordplay.
'primarily' says to take the initial letters.
'in' is an insertion indicator.
'medium strength' becomes 'esp' (I can't justify this - if you can you should give a lot more credence to this answer).
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
The first letter of 'rum' is 'r'.
'esp'+'it'='espit'
'r' inserted within 'espit' is 'ESPRIT'.
(Other definitions for esprit that I've seen before include "Vivacity, liveliness" , "What good corps needs" , "Vivacity - wit" , "Liveliness of mind or spirit" , "French morale as in de corps?" .)