They may be open to receive weapons (4)
I believe the answer is:
arms
'weapons' is the definition.
('arm' can be a synonym of 'weapon')
'they may be open to receive' is the wordplay.
'they' becomes 'ars' (I can't justify this - if you can you should believe this answer much more).
'be open' says to take the initial letters (I've seen 'opens' mean this).
'to receive' indicates putting letters inside.
The first letter of 'may' is 'm'.
'ars' going around 'm' is 'ARMS'.
Can you help me to learn more?
(Other definitions for arms that I've seen before include "Ordnance" , "Heraldic devices" , "George Bernard Shaw play '. . . . and the Man'" , "Appendages" , "Blazonry; weaponry" .)