Little item in proposal not entirely complete, not entirely pure (3-5)
I believe the answer is:
off-white
'not entirely pure' is the definition.
(not completely white)
'little item in proposal not entirely complete' is the wordplay.
'little item' becomes 'whit' (whit can mean a very small amount of something).
'in' is an insertion indicator.
'proposal' becomes 'offer' (an offer is a proposal or suggestion).
'not entirely complete' means to remove the last letter.
'offer' with its final letter taken off is 'offe'.
'whit' inserted within 'offe' is 'OFF-WHITE'.
(Other definitions for off-white that I've seen before include "Colour such as cream or bone" , "Very pale tint" , "Creamy?" , "Slightly-tinted colour" , "something like cream" .)