Hebrew, to the Spanish, is a treasure (5)
I believe the answer is:
jewel
'a treasure' is the definition.
(jewel is a kind of treasure)
'hebrew to the spanish' is the wordplay.
'hebrew' becomes 'jew' ('Jew' can be a synonym of 'hebrew').
'to the spanish' becomes 'el'.
'jew'+'el'='JEWEL'
'is' is the link.
(Other definitions for jewel that I've seen before include "(Semi) precious stone" , "Precious stone or mineral" , "Gemstone" , "The - That Was Ours (Dexter)" , "eg Ruby" .)