Sour rifleman finally got waterbird (7)
I believe the answer is:
bittern
'waterbird' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe there's a link between them I don't understand?
'sour rifleman finally' is the wordplay.
'sour' becomes 'bitter' (I've seen this before).
'finally' indicates one should take the final letters.
The final letter of 'rifleman' is 'n'.
'bitter'+'n'='BITTERN'
'got' acts as a link.
Can you help me to learn more?
(Other definitions for bittern that I've seen before include "found hidden in reed-beds" , "Bird of marshes" , "'He shall not hear the . . . cry' (Ledwidge)" , "Source of boom" , "flier" .)