Quantity of work girl shoulders beginning to polish one large mirror (4,5)

I believe the answer is:
pier glass
'large mirror' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pier glass that I've seen before include "wall mirror" , "large mirror" .)
I've seen this clue in The Times.
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