Spanish runner clutches at relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(relationship between two numbers)
'spanish runner clutches at' is the wordplay.
'spanish runner' becomes 'rio' ('river' in Spanish - a river runs).
'clutches' indicates putting letters inside.
'rio' placed around 'at' is 'RATIO'.
(Other definitions for ratio that I've seen before include "Proportion between two numbers" , "eg 1:3" , "Expression of relationship" , "Quotient of two numbers" , "Proportion, relation" .)