Quantity army doctor injected into relative (6)
I believe the answer is:
amount
'quantity' is the definition.
(I know that quantity can be written as amount)
'army doctor injected into relative' is the wordplay.
'army doctor' becomes 'MO' (medical officer).
'injected into' is an insertion indicator.
'relative' becomes 'aunt' (aunt is a type of relative).
'mo' placed into 'aunt' is 'AMOUNT'.
(Other definitions for amount that I've seen before include "Total or extent" , "Add up or be equivalent to" , "A quantity or total of anything" , "Quantity, sum" , "value" .)