Twice act for extinct animal (4)
I believe the answer is:
dodo
'extinct animal' is the definition.
'dodo' can be an answer for 'animal' (dodo is a kind of animal). I also know that 'extinct' relates to this answer.
'twice act' is the wordplay.
'twice' indicates the doubling of some letters.
'act' becomes 'do' (to do is to act or execute a task).
'do' duplicated is 'DODO'.
'for' is the link.
(Other definitions for dodo that I've seen before include "Two rooks - one was unable to fly" , "Old-fashioned person" , "Native of Mauritius" , "E [EXTINCT] bird" , "Square" .)