Demonstrator perhaps in rain (6)
I believe the answer is:
shower
I believe this is a double definition.
'demonstrator perhaps' is the first definition.
(someone who shows or demonstrates)
'rain?' is the second definition.
(I know that shower is a type of rain)
'in' acts as a link.
(Other definitions for shower that I've seen before include "Short downpour" , "bathroom fitment" , "Brief period of rain" , "an awful lot" , "Fall of rain; slack people (slang)" .)