Driver missing during hostilities (7)
I believe the answer is:
wagoner
'driver' is the definition.
(wagoner is a kind of driver)
'missing during hostilities' is the wordplay.
'missing' becomes 'gone' ('gone' can be a synonym of 'missing').
'during' means one lot of letters goes inside another.
'hostilities' becomes 'war' (I've seen this before).
'gone' inserted within 'war' is 'WAGONER'.
(Other definitions for wagoner that I've seen before include "Driver of cart" , "Carter" .)