In the river, floundering, Bertie's no end upset! (5)
I believe the answer is:
tiber
'in the river floundering' is the definition.
I can't judge whether this defines the answer.
'bertie's no end upset' is the wordplay.
'no end' means to remove the last letter.
'upset' is an anagram indicator.
'bertie' with its last letter taken away is 'berti'.
'berti' is an anagram of 'TIBER'.
Can you help me to learn more?
(Other definitions for tiber that I've seen before include "divides Rome" , "Italian river with Ostia at its mouth" , "Rome's river" , "River entering the Mediterranean at Ostia" , "Italian banker" .)