Weed in cool river tickles bathers on the bottom (14)

'bathers on the bottom' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.

'weed in cool river tickles' is the wordplay.
'weed' becomes 'pot' (synonyms).
'in' means one lot of letters goes inside another.
'cool river' becomes 'hippo' (I can't justify this - if you can you should believe this answer much more).
'tickles' becomes 'amuses' (tickle can mean to entertain or amuse).
'hippo'+'amuses'='hippoamuses'
'pot' placed within 'hippoamuses' is 'HIPPOPOTAMUSES'.

Can you help me to learn more?