Rub down with a bit of spit to begin with (4)
I believe the answer is:
sand
'rub down' is the definition.
(eg rub down with sandpaper)
'with a bit of spit to begin with' is the wordplay.
'with' becomes 'and'.
'a bit of' says to take the initial letters.
'to begin with' says to put letters next to each other.
The first letter of 'spit' is 's'.
'and' put after 's' is 'SAND'.
(Other definitions for sand that I've seen before include "It's used to make glass" , "George -, nineteenth century French novelist" , "Hourglass contents" , "Polish" , "George - - (French authoress)" .)