The round part of a doorbell (3)
I believe the answer is:
orb
'the round' is the definition.
(thesaurus)
'part of a doorbell' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ORB' is hidden within 'doorbell'.
(Other definitions for orb that I've seen before include "A sphere or globe" , "A poetic eye, bro" , "Heavenly body" , "Ornamental sphere" , "Item of royal regalia" .)