Wrong keeping head of cattle, cultivate a grain (9)
I believe the answer is:
scintilla
'grain' is the definition.
(thesaurus)
'wrong keeping head of cattle cultivate a' is the wordplay.
'wrong' becomes 'sin' (I've seen this before**).
'keeping' means one lot of letters goes inside another.
'head of' indicates taking the first letters.
'cultivate' becomes 'till' (I've seen this before).
The first letter of 'cattle' is 'c'.
'sin' placed around 'c' is 'scin'.
'scin'+'till'+'a'='SCINTILLA'
(Other definitions for scintilla that I've seen before include "Tiny hint, trace" , "A spark; a trace" , "Slight amount" , "Tiny trace or amount" , "Tiniest bit" .)