Barking heads (4)
I believe the answer is:
nuts
'heads' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for nuts that I've seen before include "Hard-shelled fruit" , "'Here we go gathering . . . . In May'" , "Mad (slang)" , "Mad - hard fruits" , "One eats these" .)