Outside bird caged by old metal (8)
I believe the answer is:
external
'outside' is the definition.
('external' is associated in meaning with 'outside')
'bird caged by old metal' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird**).
'caged by' is an insertion indicator.
'old' becomes 'ex'.
'metal' becomes 'al' (Al is an example).
'ex'+'al'='exal'
'tern' going inside 'exal' is 'EXTERNAL'.
(Other definitions for external that I've seen before include "Lying outside" , "Coming from outside" , "Surface" , "Outward" , "abroad" .)