Lie finally in hide to see some birds in flight (5)
I believe the answer is:
skein
'some birds in flight' is the definition.
(skein can mean a flock of geese)
'lie finally in hide' is the wordplay.
'finally' says to take the final letters.
'in' means one lot of letters goes inside another.
'hide' becomes 'skin' (a hide is an animal skin).
The final letter of 'lie' is 'e'.
'e' inserted within 'skin' is 'SKEIN'.
'to see' is the link.
(Other definitions for skein that I've seen before include "Clock face" , "A number of birds" , "Loosely coiled bundle of wool" , "Flock on the move" , "Coiled length of yarn" .)