Fellow diner putting hot water on tea (8)
I believe the answer is:
messmate
'fellow diner' is the definition.
(I've seen this before)
'hot water on tea' is the wordplay.
'hot water' becomes 'mess' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'tea' becomes 'mate' (South American hot drink).
'mess'+'mate'='MESSMATE'
'putting' is the link.
Can you help me to learn more?
(Other definitions for messmate that I've seen before include "He sits down with one" , "Fellow diner" , "(In the navy) someone with whom one shares communal facilities" , "Fellow soldier" , "Sharer of the same (military) accommodation" .)