Journal mostly backing military action? (4)
I believe the answer is:
raid
'military action?' is the definition.
(raid is a kind of military action)
'journal mostly backing' is the wordplay.
'journal' becomes 'diary' (diary is a kind of journal).
'mostly' means to remove the last letter.
'backing' says the letters should be written backwards.
'diary' with its final letter removed is 'diar'.
'diar' in reverse letter order is 'RAID'.
(Other definitions for raid that I've seen before include "Sudden and surprise attack" , "Foray, sudden attack" , "Surprise police visit" , "Go on the attack" , "Smash-and-grab?" .)