Black water that runs red can be a pain! (7)

I believe the answer is:
brioche
'be a pain' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'black water that runs red' is the wordplay.
I cannot really see how this works, but
'black' could be 'b' (abbreviation as in pencil hardness) and 'b' is found within the answer.
'runs' could be 'r' (cricket abbreviation) and 'r' is found within the answer.
'red' could be 'che' (Che Guevara the Marxist revolutionary) and 'che' is located in the answer.
The remaining letters 'io' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'can' acts as a link.
Can you help me to learn more?
(Other definitions for brioche that I've seen before include "Light cake-like loaf or roll" , "Hire cob (anag) - bread" , "Sponge cake or roll" , "Soft roll sometimes mixed with currants" , "Light sweetish roll" .)
