Rodent I love in relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(relationship between two numbers)
'rodent i love' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'love' becomes 'o' (love means zero in tennis).
'rat'+'i'+'o'='RATIO'
'in' acts as a link.
(Other definitions for ratio that I've seen before include "Mathematical relationship" , "Proportion between two numbers" , "Mathematical relation of one thing to another" , "Quotient, proportion" , "Relative magnitudes of two quantities" .)