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