Anatomically part of the arm (3)
I believe the answer is:
ear
'anatomically part' is the definition.
'ear' can be an answer for 'part' (I have seen 'Part of cereal' mean 'ear' so perhaps 'part' could also mean 'ear'). I'm not sure about the 'anatomically' bit.
'of the arm' is the wordplay.
'of' indicates the answer is hidden within the clue.
'EAR' is hidden in the letters of 'the arm'.
Can you help me to learn more?
(Other definitions for ear that I've seen before include "'Give every man thine ..., but few thy voice'/Hamlet" , "Fruiting head of a cereal" , "Place for a plug" , "Shell-like; corn" , "Hearing aid?" .)