British pour water on a clever person (5)
I believe the answer is:
brain
'a clever person' is the definition.
(I've seen this before)
'british pour water on' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'pour water on' becomes 'rain'.
'b'+'rain'='BRAIN'
(Other definitions for brain that I've seen before include "Mind" , "Whack on the head" , "Hit hard; intellect" , "It's in the skull" , "Seat of the intellect" .)