Incas till ground for grain (9)
I believe the answer is:
scintilla
'for grain' is the definition.
(thesaurus)
'incas till ground' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'incas'+'till'='incastill'
'incastill' with letters rearranged gives 'SCINTILLA'.
(Other definitions for scintilla that I've seen before include "Slight amount" , "Tiny hint, trace" , "scrap!" , "Tiny, scarcely detectable amount" , "The smallest amount" .)