A tango in Rio leading to relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(thesaurus)
'a tango in rio' is the wordplay.
'tango' becomes 't' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'a'+'t'='at'
'at' going within 'rio' is 'RATIO'.
'leading to' is the link.
(Other definitions for ratio that I've seen before include "E.g. two to one" , "Quotient of two numbers" , "Comparative measure" , "A trio (anag)" , "Proportion of one thing to another" .)