Driver missing in combat (7)
I believe the answer is:
wagoner
'driver' is the definition.
(wagoner is a kind of driver)
'missing in combat' is the wordplay.
'missing' becomes 'gone' ('gone' can be a synonym of 'missing').
'in' is an insertion indicator.
'combat' becomes 'war' (I've seen this in other clues).
'gone' inserted within 'war' is 'WAGONER'.
(Other definitions for wagoner that I've seen before include "Driver of cart" , "Carter" .)