Mind - first bit is wet (5)
I believe the answer is:
brain
'mind' is the definition.
(I know that mind can be written as brain)
'first bit is wet' is the wordplay.
'first bit' becomes 'b' (1st letter of 'bit').
'is wet' becomes 'rain' (wet weather).
'b'+'rain'='BRAIN'
(Other definitions for brain that I've seen before include "Control centre of nervous system" , "Mind" , "Organ of the intellect" , "Mastermind" , "to get hit over the head" .)