Security men on board are weapons experts (7-2-4)

'security men' is the definition.
'masters-at-arms' can be an answer for 'men' (master-at-arms is a kind of man). I am not certain of the 'security' bit.

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

'on' acts as a link.

Can you help me to learn more?