Clue: Bird that's coming back round initially thanks you forever (8)
Answer: ETERNITY
Explanation: Bird that's coming back round initially thanks you forever (8)
**TERN** ("bird") with **<=I.E.** ("that is", *coming back*) *round* + [*initially*] **T**(hanks) **Y**(ou), so **E(TERN)I-T.Y.