Standard portion of pudding is never rejected (6)
I believe the answer is:
ensign
'standard' is the definition.
(both can mean a flag)
'portion of pudding is never rejected' is the wordplay.
'portion of' indicates a hidden word.
'rejected' shows that the letters should be reversed in order.
'ngisne' is hidden within 'pudding is never'.
'ngisne' back-to-front is 'ENSIGN'.
(Other definitions for ensign that I've seen before include "Small flag or junior naval officer" , "Commissioned rank in some navies" , "Junior US naval officer" , "Lowly naval officer" , "SECOND PART OF 10" .)