Employ too many workers initially to erect large shelf (10)

I believe the answer is:

overmantel

'shelf' is the definition.
(I know that overmantel is a type of shelf)

'employ too many workers initially to erect large' is the wordplay.
'employ too many workers' becomes 'overman' (employ more staff than needed).
'initially' says to take the initial letters.
The initial letters of 'to erect large' is 'tel'.
'overman'+'tel'='OVERMANTEL'

(Another definition for overmantel that I've seen is " shelf with mirror".)

I've seen this clue in The Times.

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