Coming into office entrance, no-one back (9)
I believe the answer is:
accession
'coming' is the definition.
(thesaurus)
'entrance no-one back' is the wordplay.
'entrance' becomes 'access' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'back' shows that the letters should be reversed in order.
'no'+'i'='noi'
'noi' back-to-front is 'ion'.
'access'+'ion'='ACCESSION'
'into office' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for accession that I've seen before include "New item added to a library or museum" , "Act of taking up office" .)