In Bavaria, certainly one lake hot: river cooler (9)

I believe the answer is:

calaboose

I'm a little stuck... Click here to teach me more about this clue!

'hot river cooler' is the definition.
'calaboose' can be an answer for 'cooler' (thesaurus). I am unsure of the 'hot river' bit.

'in bavaria certainly one lake' is the wordplay.
'in' is an insertion indicator.
'bavaria certainly' becomes 'caboose' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'a' (a thing is one thing).
'lake' becomes 'L' (geographical abbreviation).
'a'+'l'='al'
'caboose' enclosing 'al' is 'CALABOOSE'.

Can you help me to learn more?

(Other definitions for calaboose that I've seen before include "jug" , "US jail" .)

I've seen this clue in The Guardian.

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