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' suggests removing the centre (only the extremes of the word are used).
'each' becomes 'ea' (abbreviation).
'round' is a reversal indicator.
'logical' with its centre removed is 'll'.
'ea' written backwards gives '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" .)