Prophet put back a tiny amount of weight on very hot day (9)
I believe the answer is:
zoroaster
'prophet' is the definition.
(I know that Zoroaster is a prophet)
'very hot day' is the wordplay.
'very' becomes 'zo' (I can't explain this - if you can you should give a lot more credence to this answer).
'hot day' becomes 'roaster' (I've seen this before).
'zo'+'roaster'='ZOROASTER'
'put back a tiny amount of weight on' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for zoroaster that I've seen before include "Religious leader" , "Persian prophet" .)