Still cool in river (5)

I believe the answer is:

photo

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'still' is the definition.
(I know that still can be written as photo)

'cool in river' is the wordplay.
'cool' becomes 'hot' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'river' becomes 'Po' (Italian river).
'hot' inserted into 'po' is 'PHOTO'.

Can you help me to learn more?

(Other definitions for photo that I've seen before include "Picture (abbr)" , "Snap, pic" , "Picture, in short" , "Still" , "Camera picture" .)

I've seen this clue in The Daily Mail.

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