The most common and most likely answer to this crossword clue is the 7 letter word RE-ECHO
I've seen this crossword clue in Independent
R
E
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E
C
H
O
Buddy explains!

The straight part
We know that "That’s to have added resonance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s to have added resonance = re-echo.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
RE-ECHO noun- A second or subsequent echo
- To echo again; to reverberate