Pure eating is correct (8)
I believe the answer is:
chastise
'correct' is the definition.
('chastise' can be a synonym of 'correct')
'pure eating is' is the wordplay.
'pure' becomes 'chaste' ('chaste' can be similar in meaning to 'pure').
'eating' means one lot of letters goes inside another.
'chaste' placed around 'is' is 'CHASTISE'.
(Other definitions for chastise that I've seen before include "Censure severely" , "Inflict physical corrective treatment" , "Castigate or punish" , "discipline" , "Severely reprimand" .)