Crossword Genius

Some anthems not defined in ordinal (3)

Ross

I believe the answer is:

nth

'not defined in ordinal' is the definition.
(ordinal form of n, an undefined number)

'some anthems' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'NTH' is hidden within 'anthems'.

(Other definitions for nth that I've seen before include "Umpteenth" , "To the -- degree" .)

I've seen this clue in The Independent.
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