Too old really, Labour leader admitted (6)

I believe the answer is:

overly

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

'old really labour leader admitted' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'really' becomes 'very' (I've seen this before).
'leader' indicates taking the first letters.
'admitted' indicates putting letters inside.
The initial letter of 'labour' is 'l'.
'very' placed around '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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