Speak badly of wound one can’t directly see? (8)
I believe the answer is:
backbite
'speak badly of' is the definition.
I don't understand how the remainder of the clue works.
(Other definitions for backbite that I've seen before include "Gossip maliciously" , "Speak ill of (someone) in their absence" , "Speak unkindly about someone who is absent" , "Say spiteful things about someone" , "Speak unkindly of (someone) in their absence" .)