Abstain unhappily, accommodating clever old Protestant (10)

I believe the answer is:

anabaptist

'old protestant' is the definition.
(Anabaptism is a 17th-century Christian faith)

'abstain unhappily accommodating clever' is the wordplay.
'unhappily' indicates an anagram.
'accommodating' is an insertion indicator.
'clever' becomes 'apt' ('apt' can be a synonym of 'clever').
'abstain' with letters rearranged gives 'anabist'.
'anabist' enclosing 'apt' is 'ANABAPTIST'.

(Another definition for anabaptist that I've seen is " Protestant sect member".)

I've seen this clue in The Times.

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