Respond to just a subtle trace (5)

I believe the answer is:

react

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'respond' is the definition.
(to react to something is to respond to it)

'just a subtle trace' is the wordplay.
'trace' anagrams to 'REACT'.
However, I'm not clear how this is indicated.

'to' is the link.

Can you help me to learn more?

(Other definitions for react that I've seen before include "Retaliate" , "Show opposition" , "Respond emotionally" , "provide an answer" , "Hit back" .)

I've seen this clue in the Evening Standard.

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