Prove one's humanity, in a way (3)
I believe the answer is:
err
'prove one's humanity in a way' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for err that I've seen before include "Miss the mark" , "'To . . . is human, to forgive divine'" , "Make a wrong move" , "Make mistakes" , "Drop a clanger" .)