Traitor I love shows proportion (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I know that proportion can be written as ratio)
'traitor i love' is the wordplay.
'traitor' becomes 'rat' (I've seen this before).
'love' becomes 'o' (love means zero in tennis).
'rat'+'i'+'o'='RATIO'
'shows' acts as a link.
(Other definitions for ratio that I've seen before include "Relative proportion" , "Expression of relationship" , "Mathematical relation of one thing to another" , "Eg, 4 to 3" , "2:1, for example" .)