Aquatic bird extremely eager to catch fish (5)
I believe the answer is:
eider
'aquatic bird' is the definition.
(eider is a kind of aquatic bird)
'extremely eager to catch fish' is the wordplay.
'extremely' suggests removing the centre (letters on the extremes of the word are used).
'to catch' indicates putting letters inside.
'fish' becomes 'ide' (type of fish).
'eager' with its centre removed is 'er'.
'er' placed around 'ide' is 'EIDER'.
(Other definitions for eider that I've seen before include "Source of fine soft down" , "Duck known for down" , "Reed I make into a duck" , "Species of duck" , "Down's source" .)