She comes up to us, being at a loose end (3)
I believe the answer is:
sue
'she' is the definition.
'comes up to us being at a loose end' is the wordplay.
'comes up to' is a reversal indicator (I've seen 'come up to' mean this).
'being at' says to put letters next to each other.
'end' says to take the final letters.
The final letter of 'loose' is 'e'.
'us' in reverse letter order is 'su'.
'su'+'e'='SUE'
(Other definitions for sue that I've seen before include "Use to prosecute" , "Humbly request" , "Make a legal claim" , "Take legal case against" , "Entreat; girl's name (dim.)" .)