Spanish banker clutches at relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(I've seen this in another clue)
'spanish banker clutches at' is the wordplay.
'spanish banker' becomes 'rio' (I can't justify this - if you can you should give a lot more credence to this answer).
'clutches' means one lot of letters goes inside another.
'rio' enclosing 'at' is 'RATIO'.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Relationship of numbers" , "A trio (anag)" , "One to one - maybe" , "Measure of two relative sizes" , "Comparative size" .)