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