Crossword Genius

What one needs to pick up a weapon, maybe (3)

Ross

I believe the answer is:

arm

I'm a little stuck... Click here to teach me more about this clue!

'what one needs to pick up a weapon maybe' is the definition.
I can't tell whether this defines the answer.

'one needs to pick up a weapon maybe' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'needs' means one lot of letters go next to another.
'to pick up' is a reversal indicator.
'a weapon maybe' becomes 'mr' (I can't justify this - if you can you should believe this answer much more).
'mr' reversed gives 'rm'.
'a'+'rm'='ARM'

Can you help me to learn more?

(Other definitions for arm that I've seen before include "Equip with weapon" , "Accoutre" , "Activate warhead" , "Steel" , "Supply weapons" .)

I've seen this clue in The Globe and Mail and The Irish Independent.
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