Certain ablutions queen abandoned (14)

I believe the answer is:

unquestionable

'certain' is the definition.
(thesaurus)

'ablutions queen abandoned' is the wordplay.
'abandoned' indicates an anagram.
'ablutions'+'queen'='ablutionsqueen'
'ablutionsqueen' with letters rearranged gives 'UNQUESTIONABLE'.

(Another definition for unquestionable that I've seen is " beyond doubt".)

I've seen this clue in The Telegraph.

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