Rudimentary measure of grain not complete (8)
I believe the answer is:
inchoate
'rudimentary' is the definition.
(both can mean still in development)
'measure of grain not complete' is the wordplay.
'measure' becomes 'inch' (inch is a kind of measure).
'of grain' becomes 'oaten' (made from oats).
'not complete' means to remove the last letter.
'oaten' with its last letter taken away is 'oate'.
'inch'+'oate'='INCHOATE'
(Other definitions for inchoate that I've seen before include "just beginning" , "Confused" , "at an early stage" , "Unfinished" , "Undeveloped" .)