Poem initially recalling European river (4)
I believe the answer is:
oder
'european river' is the definition.
(River Oder in central Europe)
'poem initially recalling' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'initially' suggests taking the first letters.
The first letter of 'recalling' is 'r'.
'ode'+'r'='ODER'
(Other definitions for oder that I've seen before include "School blazer insignia" , "waterway" , "European flower" , "German and Polish river" , "river, one in central Europe" .)