Part of Mediterranean an eagle has nested at regularly (6,3)
I believe the answer is:
aegean sea
'part of mediterranean' is the definition.
('Aegean Sea' is part of 'mediterranean')
'an eagle has nested at regularly' is the wordplay.
'an' becomes 'a'.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'eagle'+'has'+'nested'+'at'='eaglehasnestedat'
The alternate letters of 'eaglehasnestedat' are 'egeansea'.
'a'+'egeansea'='AEGEAN SEA'
(Other definitions for aegean sea that I've seen before include "Part of the Mediterranean between Greece and Turkey" , "Main" , "Greek island home?" , "Where 11 [GREEK] swims" , "Stretch of water between Turkey and Greece" .)