Too old? Extremely -- about fifty (6)

I believe the answer is:

overly

'too' is the definition.
('overly' can be a synonym of 'too')

'old? extremely about fifty' is the wordplay.
'old?' becomes 'o' (common abbreviation eg in OE for Old English).
'extremely' becomes 'very' (I've seen this before).
'about' indicates putting letters inside.
'fifty' becomes 'l' (roman numeral).
'very' enclosing 'l' is 'verly'.
'o'+'verly'='OVERLY'

(Another definition for overly that I've seen is " Excessively".)

I've seen this clue in The Telegraph.

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