Hidden observer coming round about in pursuit of ordinary bird (6)
I believe the answer is:
osprey
'bird' is the definition.
(osprey is a kind of bird)
'hidden observer coming round about in pursuit of ordinary' is the wordplay.
'hidden observer' becomes 'spy' (a spy secretly observes things).
'coming round' is an insertion indicator.
'about' becomes 're' (regarding).
'in pursuit of' means one lot of letters go next to another (some letters chase after others).
'ordinary' becomes 'O' (abbreviation eg in O-Level, Ordinary Level).
'spy' enclosing 're' is 'sprey'.
'sprey' put after 'o' is 'OSPREY'.
(Other definitions for osprey that I've seen before include "Fish-eater" , "Fish-eating hawk" , "Large bird that preys on fish in inland waters" , "Bird of fine feather" , "Large bird preying on fish in inland waters" .)