Double act that's now extinct (4)
I believe the answer is:
dodo
'now extinct' is the definition.
I know that 'extinct' relates to this answer.
'double act' is the wordplay.
'double' shows that letters should be duplicated.
'act' becomes 'do' (to do is to act or execute a task).
'do' duplicated is 'DODO'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for dodo that I've seen before include "Old-fashioned person" , "Square" , "E [EXTINCT] bird" , "bird long dead" , "Flightless bird, extinct since the 17th century" .)