Cookie containing odd bits of grain and herb (7)
I believe the answer is:
oregano
'herb' is the definition.
(oregano is a kind of herb)
'cookie containing odd bits of grain' is the wordplay.
'cookie' becomes 'oreo cookie' (brand of cookie).
'containing' indicates putting letters inside.
'odd bits of' indicates alternate letters.
The alternating letters of 'grain' are 'gan'.
'oreo' going around 'gan' is 'OREGANO'.
'and' acts as a link.
(Other definitions for oregano that I've seen before include "Aromatic culinary herb" , "aromatic plant" , "Marjoram-like herb" , "Pizza herb" , "food flavouring" .)