Different water temperatures crossed by returning river boat (5)
I believe the answer is:
yacht
'boat' is the definition.
(I know that yacht is a type of motorboat)
'different water temperatures crossed by returning river' is the wordplay.
'different water temperatures' becomes 'ch' (I can't explain this - if you can you should believe this answer much more).
'crossed by' is an insertion indicator (inserted letters are crossed or covered by others).
'returning' shows that the letters should be reversed in order.
'river' becomes 'tay' (Scottish river).
'tay' reversed gives 'yat'.
'ch' inserted inside 'yat' is 'YACHT'.
Can you help me to learn more?
(Other definitions for yacht that I've seen before include "Pleasure craft" , "Light sailing-vessel for sport" , "Sailing craft" , "Sailing boat" , "Kind of boat" .)