Ruin is complete (5)
I believe the answer is:
total
I believe this is a double definition.
'ruin' is the first definition.
(total can mean to totally ruin something)
'complete' is the second definition.
(similar in meaning)
'is' is the link.
(Other definitions for total that I've seen before include "outright" , "Completely destroy" , "to give the final figure" , "Not qualified" , "Entirety" .)