NB regular treatment needed for part of arm (3,6)
I believe the answer is:
gun barrel
'part of arm' is the definition.
(I know this)
'nb regular treatment' is the wordplay.
'treatment' indicates anagramming the letters (letters treated or processed differently).
'nb'+'regular'='nbregular'
'nbregular' anagrammed gives 'GUN BARREL'.
'needed for' acts as a link.
(Other definitions for gun barrel that I've seen before include "Part of weapon" , "Where a slug follows" , "Stock attachment" , "a proverbially straight path?" , "bore, to be absolutely straight" .)