Novel reflection about a small river bird (4)
I believe the answer is:
wren
'bird' is the definition.
(wren is a kind of bird)
'novel reflection about a small river' is the wordplay.
'novel' becomes 'new' (novel means new or fresh).
'reflection' is a reversal indicator.
'about' is an insertion indicator.
'a small river' becomes 'r' (abbreviation for river; 'small' asks for an abbreviation).
'new' written backwards gives 'wen'.
'wen' enclosing 'r' is 'WREN'.
(Other definitions for wren that I've seen before include "Small active brown northern hemisphere bird" , "A songster" , "Sir Christopher -, architect of St Paul's" , "Very small brown songbird (4)" , "Architect of St Paul's Cathedral" .)