Pub welcomes the one who washes (6)
I believe the answer is:
bather
'one who washes' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'pub welcomes the' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'welcomes' is an insertion indicator (in sense of 'invites in').
'bar' going around 'the' is 'BATHER'.
Can you help me to learn more?
(Other definitions for bather that I've seen before include "one getting into the drink" , "Seaside swimmer" , "man getting soaked?" , "One dipping in the briny?" .)