Problem about pet entangled in partition (6)
I believe the answer is:
septum
'partition' is the definition.
(I know that septum is a type of partition)
'problem about pet entangled' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'about' means one lot of letters goes inside another.
'entangled' indicates an anagram.
'pet' is an anagram of 'ept'.
'sum' enclosing 'ept' is 'SEPTUM'.
'in' is the link.
(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" .)