Crossword Genius

Most hostile piece, Queen, used in check (9)

Ross

I believe the answer is:

bitterest

'most hostile' is the definition.
(bitter can mean hostile or unfriendly)

'piece queen used in check' is the wordplay.
'piece' becomes 'bit' (synonyms).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'used in' is an insertion indicator.
'check' becomes 'test' (as in testing or checking something works).
'er' placed inside 'test' is 'terest'.
'bit'+'terest'='BITTEREST'

(Another definition for bitterest that I've seen is " Most cruel".)

I've seen this clue in The Telegraph.
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