500 married in Scots town's important event (8)
I believe the answer is:
landmark
'important event' is the definition.
'500 married in scots town's' is the wordplay.
'500' becomes 'd' ('d' can be a synonym of '500').
'married' becomes 'm'.
'in' indicates putting letters inside.
'scots town' becomes 'lanark' (I've seen this before).
'd'+'m'='dm'
'dm' put within 'lanark' is 'LANDMARK'.
(Other definitions for landmark that I've seen before include "Shard, for example" , "Easily identifiable monument, for example" , "Definitive event" , "Turning point, watershed" , "Conspicuous feature" .)