Duck's sign of hesitation about fish (5)
I believe the answer is:
eider
'duck's' is the definition.
(eider is a kind of duck)
'sign of hesitation about fish' is the wordplay.
'sign of hesitation' becomes 'er'.
'about' says to put letters next to each other.
'fish' becomes 'eid' (I am not sure about this - if you are sure you should believe this answer much more).
'er' after 'eid' is 'EIDER'.
Can you help me to learn more?
(Other definitions for eider that I've seen before include "Large sea duck" , "Sea bird" , "Species of duck" , "Northern sea duck" , "Duck known for down" .)