It's concluded and tied up in Rostrevor (4)
I believe the answer is:
over
'it's concluded' is the definition.
(I know this)
'up in rostrevor' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'in' indicates a hidden word.
'revo' can be found hidden inside 'rostrevor'.
'revo' reversed gives 'OVER'.
'and tied' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for over that I've seen before include "In the sky?" , "Work out of hours" , "Division of play in cricket" , "Past, surplus" , "Finished - across - in excess" .)