The mark of an illiterate half-breed (5)

I believe the answer is:

cross

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This could be a double definition.

'the mark' is the first definition.
(synonyms)

'an illiterate half-breed' is the second definition.
'cross' can be an answer for 'half-breed' (thesaurus). I am not sure about the 'an illiterate' bit.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for cross that I've seen before include "Burden" , "tangelo for one" , "Bad-tempered" , "Mark; hybrid" , "Rood - intersect - annoyed" .)

I've seen this clue in The Irish Independent.

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