Naval policemen are experts with weapons (7-2-4)

'naval policemen' is the definition.
'masters at arms' can be an answer for 'policemen' (I've seen this before). I am not sure about the 'naval' bit.

'experts with weapons' is the wordplay.
I cannot quite understand how this works, but
'experts' could be 'masters' (master is a kind of expert) and 'masters' is found within the answer.
'weapons' could be 'arms' ('arm' can be a synonym of 'weapon') and 'arms' is found in the answer.
The remaining letters 'at' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'are' acts as a link.

Can you help me to learn more?