Being ruthless in completing the countdown (8,2,7)

I believe the answer is:

stopping at nothing

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

'being ruthless' is the definition.
Both the definition and answer are adjectives.
Maybe there's a link between them I don't understand?

'in completing the countdown' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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