County Hall's opening cancelled on order of Crown (8)
I believe the answer is:
cornwall
'county' is the definition.
(I know that Cornwall is a county)
'on order of crown' is the wordplay.
'on' means one lot of letters go next to another.
'order' becomes 'all' (I can't justify this - if you can you should believe this answer much more).
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'crown' anagrammed gives 'cornw'.
'all' put after 'cornw' is 'CORNWALL'.
'hall's opening cancelled' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for cornwall that I've seen before include "Duchy" , "Land" , "Part of 1 25 [KING LEAR]" , "County of south-west England" , "County in southwestern England" .)