Holes in one's body! (7)
I believe the answer is:
sinuses
'holes' is the definition.
The answer and definition can be both body parts as well as being plural nouns.
Perhaps you can see an association between them that I can't see?
'in one's body' is the wordplay.
I cannot really understand how this works, but
'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sinuses that I've seen before include "Top spaces" , "holes in the head" , "Empty spaces" , "Cavities in skull connecting with nostrils" .)