Weapons are ready, mounted separately at first (4)
I believe the answer is:
arms
'weapons' is the definition.
('arm' can be a synonym of 'weapon')
'are ready mounted separately at first' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'are ready mounted separately' is 'ARMS'.
(Other definitions for arms that I've seen before include "See 12" , "bombs and guns etc" , "Members" , "See 11" , "Arsenal's items?" .)