Empty manhole opening behind Charlie, a long way (10)

I believe the answer is:

megaparsec

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'empty' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'manhole opening behind charlie a long way' is the wordplay.
'opening' suggests taking the first letters.
'behind charlie' becomes 'eg' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'long way' becomes 'parsec' (I've seen this before).
The initial letter of 'manhole' is 'm'.
'm'+'eg'+'a'+'parsec'='MEGAPARSEC'

Can you help me to learn more?

(Another definition for megaparsec that I've seen is " huge distance".)

I've seen this clue in The Independent.

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