Rob can rely on grinding grain (10)
I believe the answer is:
barleycorn
'grain' is the definition.
(I know that barleycorn is a type of grain)
'rob can rely on grinding' is the wordplay.
'on' is a charade indicator (letters next to each other).
'grinding' indicates anagramming the letters (letters must be ground into a new form).
'can'+'rely'='canrely'
'rob' put after 'canrely' is 'canrelyrob'.
'canrelyrob' anagrammed gives 'BARLEYCORN'.
(Other definitions for barleycorn that I've seen before include "Grain" , "It was used to measure" .)