Great number into working on both sides of trench (8)

I believe the answer is:

trillion

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'great number' is the definition.
(very large number)

I don't understand how the remainder of the clue works.

(Other definitions for trillion that I've seen before include "Very large power of 10" , "1 and 18 zeros" , "US billion" , "enormous figure" , "A million multiplied by twice itself" .)

I've seen this clue in The Times.

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