Two bits left smoother (3)
I believe the answer is:
oil
'smoother' is the definition.
(eg something which smooths a machine's workings)
'two bits left' is the wordplay.
'two bits' means two substitutions for 'bit'.
'bit' becomes 'o' (resembles 0, a binary digit or bit).
'bit' becomes 'i' (resembles 1, a binary digit or bit).
'left' becomes 'L' (common abbreviation).
'o'+'i'+'l'='OIL'
(Other definitions for oil that I've seen before include "Fuel, lubricant" , "Lubricating liquid" , "Liquid gold" , "Lubricant or petroleum" , "Fluid lubricant" .)