She'll back us to the echo! (3)
I believe the answer is:
sue
'she'll' is the definition.
I can't tell whether this definition defines the answer.
'back us to the echo' is the wordplay.
'back' says the letters should be written backwards.
'to the echo' becomes 'e' (phonetic alphabet: alpha, bravo, charlie etc.).
'us' backwards is 'su'.
'su'+'e'='SUE'
Can you help me to learn more?
(Other definitions for sue that I've seen before include "Bring a legal action" , "Appeal" , "Take legal action against (someone), girl's name" , "Susan (dim.)" , "Take court action" .)