Rotter I love displays proportion (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I've seen this before)
'rotter i love' is the wordplay.
'rotter' becomes 'rat' (synonyms).
'i love' becomes 'io' (I've seen this in other clues).
'rat'+'io'='RATIO'
'displays' is the link.
(Other definitions for ratio that I've seen before include "Mathematical relation of one thing to another" , "3 relation" , "Relative proportion" , "E.g. sin i" , "Expression of relationship" .)