Possessed by monster, went down and broke up (8)
I believe the answer is:
divorced
'broke up' is the definition.
(divorcing is a kind of breaking up)
'possessed by monster went down' is the wordplay.
'possessed by' means one lot of letters goes inside another.
'monster' becomes 'orc' (fantasy creature).
'went down' becomes 'dived' (diving is a kind of going down).
'orc' put inside 'dived' is 'DIVORCED'.
'and' is the link.
(Other definitions for divorced that I've seen before include "separated" , "disbanded union" , "Of marriage, legally terminated" , "No longer married" , "Of married couple, legally parted" .)