Period historian initially does once (4)

I believe the answer is:

doth

'does once' is the definition.
(I've seen this in another clue)

'period historian initially' is the wordplay.
'period' becomes 'dot' (as in a full stop).
'initially' suggests taking the first letters.
The initial letter of 'historian' is 'h'.
'dot'+'h'='DOTH'

I've seen this clue in The Times.

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