Wholesome eating, eating top-class grain occasionally (10)
I believe the answer is:
nourishing
'wholesome' is the definition.
(similar in meaning)
'eating eating top-class grain occasionally' is the wordplay.
'eating' becomes 'noshing' (noshing is a kind of eating).
'eating' is an insertion indicator.
'top-class' becomes 'U' (as in U and non-U speech).
'occasionally' indicates alternate letters.
The alternate letters of 'grain' are 'ri'.
'u'+'ri'='uri'
'noshing' placed around 'uri' is 'NOURISHING'.
(Other definitions for nourishing that I've seen before include "Nutritious" , "nutritive" .)