Port is the ruination of one? Harsh, IMO! (9)
I believe the answer is:
hiroshima
'port' is the definition.
(I know that Hiroshima is a port)
'the ruination of one? harsh imo' is the wordplay.
'the ruination of' is an anagram indicator (I've seen this in other clues).
'one?' becomes 'i' (Roman numeral).
'i'+'harsh'+'imo'='iharshimo'
'iharshimo' is an anagram of 'HIROSHIMA'.
'is' acts as a link.
(Other definitions for hiroshima that I've seen before include "Japanese city" , "First atomic bomb target" , "First city to be hit by atomic bomb" , "Honshu City" , "Japanese port on Honshu island" .)