Coming from moor, bitterns fly in a circle (5)
I believe the answer is:
orbit
'fly in a circle' is the definition.
(I've seen this before)
'coming from moor bitterns' is the wordplay.
'coming from' says the answer is hidden in the clue.
'ORBIT' is hidden within 'moor bitterns'.
(Other definitions for orbit that I've seen before include "Planet's path" , "heavenly passage" , "Stellar path" , "Circle in space" , "astronauts' course" .)