Very hot current round mouth of river (8)
I believe the answer is:
tropical
'very hot' is the definition.
(in the tropics)
'current round mouth of river' is the wordplay.
'current' becomes 'topical' (similar in meaning).
'round' indicates putting letters inside.
'mouth of river' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'topical' going around 'r' is 'TROPICAL'.
Can you help me to learn more?
(Other definitions for tropical that I've seen before include "Relating to very warm areas" , "Boiling" , "Hot and humid, near the equator" , "it could be too hot" , "Hot and humid type of weather" .)