Swear to confine working concern to transit hall (9)
I believe the answer is:
concourse
'transit hall' is the definition.
'concourse' can be an answer for 'hall' (concourse is a kind of hall). I am unsure of the 'transit' bit.
'swear to confine working concern' is the wordplay.
'swear' becomes 'curse' (synonyms).
'to confine' is an insertion indicator.
'working concern' becomes 'onco' (I can't justify this - if you can you should believe this answer much more).
'curse' enclosing 'onco' is 'CONCOURSE'.
'to' is the link.
Can you help me to learn more?
(Other definitions for concourse that I've seen before include "crowd" , "Meeting place" , "part of airport" , "Crowds" , "Open area in or outside a public building" .)