Aircraftman leaves in two litres of milk (7)
I believe the answer is:
lacteal
'of milk' is the definition.
(lacteal means relating to milk)
'aircraftman leaves in two litres' is the wordplay.
'two litres' means two replacements for 'litre'.
'aircraftman' becomes 'ac' (military abbreviation).
'leaves' becomes 'tea' (as in tea leaves).
'in' is an insertion indicator.
'litre' becomes 'L' (abbreviation).
'litre' becomes 'L' (abbreviation).
'ac'+'tea'='actea'
'l'+'l'='ll'
'actea' inserted within 'll' is 'LACTEAL'.
(Other definitions for lacteal that I've seen before include "Milk-like; pertaining to milk" , "Milky resin" .)