Absolute ruin (5)
I believe the answer is:
total
I believe this clue is a double definition.
'absolute' is the first definition.
(similar in meaning)
'ruin' is the second definition.
(total can mean to totally ruin something)
(Other definitions for total that I've seen before include "Completely destroy" , "outright" , "The whole, sum" , "French oil and gas company" , "Aggregate, sum" .)