Einstein in British downpour (5)
I believe the answer is:
brain
'einstein' is the definition.
(I know this)
'british downpour' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'downpour' becomes 'rain' (I've seen this in another clue).
'b'+'rain'='BRAIN'
'in' is the link.
(Other definitions for brain that I've seen before include "Seat of the intellect" , "Haemorrhaging of talent" , "The grey matter" , "Clever person" , "Clever type" .)