Security man on board is a weapons expert (6-2-4)
I believe the answer is:
master-at-arms
'security man' is the definition.
'master-at-arms' can be an answer for 'man' (master-at-arms is a kind of man). I am unsure of the 'security' bit.
'on board is a weapons expert' is the wordplay.
I cannot really understand how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is found within the answer.
'a' is present in the answer.
'weapons' could be 'arms' ('arm' can be a synonym of 'weapon') and 'arms' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for master-at-arms that I've seen before include "Ship's chief police officer" , "Naval chief petty officer" , "Chief Petty Officer in charge of discipline" , "enforcer of naval discipline" , "Senior naval petty officer responsible for discipline" .)