Dividing partition between the nostrils (6)
I believe the answer is:
septum
'dividing partition between the nostrils' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for septum that I've seen before include "Dividing partition between the nostrils" , "Partition in nose between nostrils" , "Partition between two cavities of the nose" , "Division between nostrils" , "Partition between two chambers" .)