Attack journal mostly written up (4)
I believe the answer is:
raid
'attack' is the definition.
(raid is a kind of attack)
'journal mostly written up' is the wordplay.
'journal' becomes 'diary' (diary is a kind of journal).
'mostly' means to remove the last letter.
'written up' is a reversal indicator.
'diary' with its last letter removed is 'diar'.
'diar' back-to-front is 'RAID'.
(Other definitions for raid that I've seen before include "bust" , "Sally" , "Go on the attack" , "Assault, attack" , "Stock Exchange activity" .)