Crossword Genius

Prove aunt is best at rambling (12)

Ross

I believe the answer is:

substantiate

'prove' is the definition.
(thesaurus)

'aunt is best at rambling' is the wordplay.
'rambling' indicates anagramming the letters.
'aunt'+'is'+'best'+'at'='auntisbestat'
'auntisbestat' anagrammed gives 'SUBSTANTIATE'.

(Other definitions for substantiate that I've seen before include "Confirm" , "Prove truth of (charge)" , "Prove to be true" .)

I've seen this clue in The Independent.
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