Birds not usually seen around the river (5)
I believe the answer is:
ernes
'birds' is the definition.
(erne is a kind of bird)
'not usually seen around the river' is the wordplay.
'not usually' indicates an anagram (an unusual ordering of the letters).
'around' is an insertion indicator.
'the river' becomes 'r' (abbreviation for river).
'seen' with letters rearranged gives 'enes'.
'enes' going around 'r' is 'ERNES'.
(Other definitions for ernes that I've seen before include "sea eagles" , "Certain birds" , "Irish lough's" , "Sea-eagles" , "They fly across the sea" .)