Old bird on a lake outside (8)
I believe the answer is:
external
'outside' is the definition.
(I know that outside can be written as external)
'old bird on a lake' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'bird' becomes 'tern' (tern is a kind of bird).
'on' says to put letters next to each other.
'lake' becomes 'L' (geographical abbreviation).
'ex'+'tern'+'a'+'l'='EXTERNAL'
(Other definitions for external that I've seen before include "Relating to the outside" , "abroad" , "Outer" , "involving foreign nations" , "Belonging to the outside" .)