Man's situation is as heir ruined (5,3)
I believe the answer is:
irish sea
'man's situation' is the definition.
(the Isle of Man is in the Irish Sea)
'is as heir ruined' is the wordplay.
'ruined' indicates an anagram.
'is'+'as'+'heir'='isasheir'
'isasheir' anagrammed gives 'IRISH SEA'.
(Other definitions for irish sea that I've seen before include "Waters separating Ireland from England and Wales" , "Water around Isle of Man" , "Local arm of Atlantic" , "Waters around IOM" , "Man located here" .)