In a piece of poetry, American river (4)
I believe the answer is:
ouse
'river' is the definition.
(Yorkshire river)
'in a piece of poetry american' is the wordplay.
'in' indicates putting letters inside.
'a piece of poetry' becomes 'oe' (I am not sure about this - if you are sure you should believe this answer much more).
'american' becomes 'US' (abbreviation for United States).
'oe' placed around 'us' is 'OUSE'.
Can you help me to learn more?
(Other definitions for ouse that I've seen before include "River of 1 across" , "Lewes's river" , "Name of several English rivers" , "N English river (and S English river)" , "Name of river in Sussex or Yorkshire" .)