Relationship with a model in port (5)
I believe the answer is:
ratio
'relationship' is the definition.
(I've seen this in another clue)
'a model in port' is the wordplay.
'model' becomes 't' (Ford Model T car).
'in' is an insertion indicator.
'port' becomes 'rio' (Rio de Janeiro is a port).
'a'+'t'='at'
'at' going inside 'rio' is 'RATIO'.
'with' is the link.
(Other definitions for ratio that I've seen before include "Relative magnitudes of two quantities" , "Relationship of numbers" , "Comparative size" , "Proportion, relationship" , "nine-to-five, say" .)