Immobilise a number (3)

I believe the answer is:

This clue is a double definition.

'immobilise' is the first definition.
(as in to pin someone down)

'a number' is the second definition.
(I know that PIN is a type of number)

(Other definitions for pin that I've seen before include "Fastener; skittle" , "Small object to fasten or attach" , "Small brooch; skittle" , "Debit-card number" , "Sewer's aid" .)

I've seen this clue in the Evening Standard.

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