One bird or another crossing river with duck (5)
I believe the answer is:
heron
'one bird' is the definition.
(heron is a kind of bird)
'or another crossing river with duck' is the wordplay.
'or another' becomes 'hen' (I can't explain this - if you can you should believe this answer much more).
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'with' says to put letters next to each other.
'duck' becomes 'o'.
'r'+'o'='ro'
'hen' enclosing 'ro' is 'HERON'.
(Other definitions for heron that I've seen before include "Wading bird with long neck and legs" , "Grey or white long-legged wading bird" , "Long-legged long-necked wading bird" , "Big bird" , "Tall wading bird" .)