Bone in Dublin Bay occasionally (4)
I believe the answer is:
ulna
'bone' is the definition.
(ulna is a kind of bone)
'dublin bay occasionally' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'dublin'+'bay'='dublinbay'
The alternating letters of 'dublinbay' are 'ULNA'.
'in' is the link.
(Other definitions for ulna that I've seen before include "Long bone in the arm" , "part of anatomy" , "Thinner of two long bones in forearm" , "Part of bone-structure" , "Part of body" .)