With a redhead, I'm on edge (3)
I believe the answer is:
rim
'edge' is the definition.
(I know that rim is a type of edge)
'with a redhead i'm' is the wordplay.
'with' says to put letters next to each other.
'a redhead' becomes 'r' (head letter of 'red').
'i'm' becomes 'im' ().
'r' next to 'im' is 'RIM'.
'on' acts as a link.
(Other definitions for rim that I've seen before include "Top edge" , "Circular outer edge" , "'Brim, lip (3)'" , "An edge, canyon or cup" , "Edge - lip" .)