Tough? Oh no! (3)

I believe the answer is:

tug

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'no' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

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

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(Other definitions for tug that I've seen before include "Pull suddenly; boat" , "Pull forcibly" , "Pull sharply" , "Small boat for towing others" , "Tow (of war?)" .)

I've seen this clue in the Evening Standard.

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