Old seabird's regularly able to get outside (8)
I believe the answer is:
Here is my best explanation:
'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'.
'to get' is the link.