Bird not previously observed to soar round river (4)
I believe the answer is:
wren
'bird' is the definition.
(I know that wren is a type of passeriform bird)
'not previously observed to soar round river' is the wordplay.
'not previously observed' becomes 'new' (not seen before).
'to soar' says the letters should be written backwards (soar backwards).
'round' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'new' backwards is 'wen'.
'wen' going around 'r' is 'WREN'.
(Other definitions for wren that I've seen before include "one leaving evidence all around London monument" , "Member of the former Women's Royal Naval Service" , "Small British bird" , "Bird - former British servicewoman" , "17th Century architect" .)