A honeyeater in perpetuity (3)
I believe the answer is:
tui
'a honeyeater' is the definition.
(type of bird)
'in perpetuity' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TUI' can be found hidden inside 'perpetuity'.
(Other definitions for tui that I've seen before include "New Zealand native" , "NZ honeyeater" , "NZ bird" , "New Zealand honey-guide" , "New Zealand honeyeater" .)