Northern diocese in the United Nations not noticed (6)
I believe the answer is:
unseen
'not noticed' is the definition.
(I've seen this before)
'northern diocese in the united nations' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'diocese' becomes 'see' (I've seen this in another clue).
'in the' means one lot of letters goes inside another.
'united nations' becomes 'UN' (synonyms).
'n'+'see'='nsee'
'nsee' going inside 'un' is 'UNSEEN'.
(Other definitions for unseen that I've seen before include "not processed by pupils?" , "not spotted" , "Lurking" , "Not perceived or observed" , "Invisible or not observed" .)