... substitute messes it up, edgy about defending (8)
I believe the answer is:
deputise
'substitute' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how one could define the other.
'messes it up edgy about defending' is the wordplay.
'DEPUTISE' is hidden back-to-front within the letters.
But, I'm unsure how this is indicated.
Can you help me to learn more?
(Other definitions for deputise that I've seen before include "Appoint as stand-in" , "Act as delegate for a person" , "Appoint as a substitute" , "Appoint as representative" , "Stand in for" .)