Miles must leave to dispatch shipment (7)
I believe the answer is:
transit
'shipment' is the definition.
(transit can mean the process of shipping)
'miles must leave to dispatch' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'must leave' indicates named letters should be taken away.
'to dispatch' becomes 'transmit'.
'transmit' with 'm' removed is 'TRANSIT'.
(Other definitions for transit that I've seen before include "Pass across" , "Moving" , "The carrying of people etc" , "Crossing - passage" , "Haulage" .)