Crossword Genius

Small amount (of rice or truth) (5)

Ross

I believe the answer is:

grain

(Other definitions for grain that I've seen before include "Small piece of sand" , "Smallest measure of weight in avoirdupois" , "little salty bit" , "Silo contents" , "Particle of e.g. sand" .)

I've seen this clue in The Guardian.
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