Senior NCO on board is a weapons expert (6-2-4)
I believe the answer is:
master-at-arms
'expert' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'senior nco on board is a weapons' 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 located in the answer.
'a' is within 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 "Naval chief petty officer" , "enforcer of naval discipline" , "Ship's chief police officer" , "Senior naval petty officer responsible for discipline" , "Chief Petty Officer in charge of discipline" .)