An indication of crass ignorance (4)

I believe the answer is:

sign

'an indication' is the definition.
(thesaurus)

'of crass ignorance' is the wordplay.
'of' says the answer is hidden in the clue.
'SIGN' is hidden in the letters of 'crass ignorance'.

(Other definitions for sign that I've seen before include "Autograph" , "Notice; omen" , "Manifestation" , "Token - symbol - omen - write one's name" , "Show" .)

I've seen this clue in the Evening Standard.

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