The measure of a president, to the Spanish (6)
I believe the answer is:
bushel
'the measure' is the definition.
(bushel is a kind of measure)
'a president to the spanish' is the wordplay.
'a president' becomes 'bush' (I've seen this in other clues).
'to the spanish' becomes 'el'.
'bush'+'el'='BUSHEL'
'of' is the link.
(Other definitions for bushel that I've seen before include "Four pecks" , "Old grain measure" , "Old measure of capacity" , "Obsolete measure (equal to 8 gallons)" , "Dry measure of eight gallons" .)