There’s defence in it (5)

I believe the answer is:

depth

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

I can't explain the remainder of the clue.

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(Other definitions for depth that I've seen before include "Deepness" , "Thoroughness" , "Intensity; profundity" , "Profoundness" , "Intensity (of emotion)" .)

I've seen this clue in the Evening Standard.

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