Branch to which Umbrian occasionally returns (3)
I believe the answer is:
arm
'branch' is the definition.
(synonyms)
'umbrian occasionally returns' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'returns' says the letters should be written backwards.
The alternate letters of 'umbrian' are 'mra'.
'mra' back-to-front is 'ARM'.
'to which' is the link.
(Other definitions for arm that I've seen before include "Provide with weapons" , "Prime" , "inlet" , "Activate warhead" , "Test of strength with elbows on the table" .)