Bertie, endlessly floundering in the water (5)
I believe the answer is:
tiber
'the water' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?
'bertie endlessly floundering' is the wordplay.
'endlessly' means to remove the last letter.
'floundering' indicates anagramming the letters.
'bertie' with its final letter removed is 'berti'.
'berti' with letters rearranged gives 'TIBER'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for tiber that I've seen before include "divides Rome" , "Roman river" , "Italian banker" , "River of Rome" , "Rome's river" .)