Bones in head extremely logical, each round (8)
I believe the answer is:
patellae
'bones' is the definition.
(patella is a kind of bone)
'head extremely logical each round' is the wordplay.
'head' becomes 'pate' (humorous term for the head).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'each' becomes 'ea' (abbreviation).
'round' shows that the letters should be reversed in order.
'logical' with its centre removed is 'll'.
'ea' back-to-front is 'ae'.
'pate'+'ll'+'ae'='PATELLAE'
'in' is the link.
(Other definitions for patellae that I've seen before include "Bony bits" , "The formal kneecaps" , "Bones" , "Late leap twisted kneecaps" .)