Double act that's no longer with us (4)
I believe the answer is:
dodo
'that's no longer with us' is the definition.
(extinct bird)
'double act' is the wordplay.
'double' says to double the letters.
'act' becomes 'do' (to do is to act or execute a task).
'do' doubled is 'DODO'.
(Other definitions for dodo that I've seen before include "Old bird" , "Mauritian bird no more" , "Old fogey" , "one finished?" , "Extinct bird of Mauritius, very dead" .)