Make excellent at packing grain (9)
I believe the answer is:
fabricate
'make' is the definition.
(fabricating is a kind of making)
'excellent at packing grain' is the wordplay.
'excellent' becomes 'fab' (slang abbreviation of fabulous).
'packing' indicates putting letters inside.
'grain' becomes 'rice' (rice is a kind of grain**).
'at' placed inside 'rice' is 'ricate'.
'fab'+'ricate'='FABRICATE'
(Other definitions for fabricate that I've seen before include "Invent or manufacture a story" , "forge" , "Make up; manufacture" , "Construct or invent" , "Tell lies" .)