Old seabird's regularly able to get outside (8)
I believe the answer is:
external
'outside' is the definition.
('external' is associated in meaning with 'outside')
'old seabird's regularly able' is the wordplay.
'old' becomes 'ex'.
'seabird' becomes 'tern' (I've seen this before).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'able' are 'al'.
'ex'+'tern'+'al'='EXTERNAL'
'to get' is the link.
(Other definitions for external that I've seen before include "Relating to the outside" , "involving foreign nations" , "Outer" , "Coming from outside" , "Lying outside" .)