Perform act for an old bird (4)
I believe the answer is:
dodo
'an old bird' is the definition.
(I've seen this before)
'perform act' is the wordplay.
'perform' becomes 'do' (synonyms).
'act' becomes 'do' (to do is to act or execute a task).
'do'+'do'='DODO'
'for' is the link.
(Other definitions for dodo that I've seen before include "Square" , "Ultra-conservative" , "Extinct bird of Mauritius, very dead" , "Proverbially extinct bird" , "Large extinct bird" .)