Missive about variable architectural feature (8)

I believe the answer is:

epistyle

'architectural feature' is the definition.
(type of architrave)

'missive about variable' is the wordplay.
'missive' becomes 'epistle' (epistle is a kind of missive).
'about' means one lot of letters goes inside another.
'variable' becomes 'y' (algebra).
'epistle' enclosing 'y' is 'EPISTYLE'.

I've seen this clue in The Times.

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