Mechanism for measuring crimes blocked by police, essentially (8)

I believe the answer is:

calipers

I'm a little stuck... Click here to teach me more about this clue!

'mechanism for measuring' is the definition.
I can't judge whether this defines the answer.

'crimes blocked by police essentially' is the wordplay.
'crimes' becomes 'capers' (caper is a kind of crime).
'blocked by' means one lot of letters goes inside another (block can mean to plug a gap in something).
'essentially' means to look at the middle letters.
The middle letters of 'police' are 'li'.
'capers' going around 'li' is 'CALIPERS'.

Can you help me to learn more?

(Another definition for calipers that I've seen is " Compasses with legs".)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app