Phonograph part
'phonograph part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arm that I've seen before include "Division of company" , "Ram into limb" , "Body part" , "What soldiers do" , "Shoulder-to-wrist member" .)
'phonograph part' is the definition.
('tone arm' is part of 'phonograph')
This is all the clue.
(Another definition for tone arm that I've seen is " Location of pick-up".)