Old friend keeping aquatic bird outside (10)
I believe the answer is:
externally
'outside' is the definition.
(I've seen this before)
'old friend keeping aquatic bird' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'friend' becomes 'ally' (an ally is a friend or supporter).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'aquatic bird' becomes 'tern' (tern is a kind of aquatic bird).
'ex'+'ally'='exally'
'exally' going around 'tern' is 'EXTERNALLY'.
(Other definitions for externally that I've seen before include "outside" , "Outwardly" .)