Series less sensitive to quantity (5,6)

I believe the answer is:

round number

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'series' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'less sensitive to quantity' is the wordplay.
'less sensitive' becomes 'number' (numb means insensitive or unfeeling).
'to' means one lot of letters go next to another (I've seen this in other clues).
'quantity' becomes 'round' (round is a kind of quantity).
'number' put after 'round' is 'ROUND NUMBER'.

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I've seen this clue in The Mirror.

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