She can provide us with the key (3)
I believe the answer is:
sue
'she' is the definition.
'us with the key' is the wordplay.
'us' becomes 'su' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'the key' becomes 'e' (a musical key).
'su'+'e'='SUE'
'can provide' acts as a link.
Can you help me to learn more?
(Other definitions for sue that I've seen before include "Take legal action against (someone), girl's name" , "Bring charges against - girl" , "Susan (dim.)" , "Begin legal proceedings against" , "Seek damages" .)