I find design by the Spanish heathen (7)
I believe the answer is:
infidel
'heathen' is the definition.
(I know that heathen can be written as infidel)
'i find design by the spanish' is the wordplay.
'design' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'the spanish' becomes 'el' ('the' in Spanish).
'find' anagrammed gives 'nfid'.
'i'+'nfid'+'el'='INFIDEL'
(Other definitions for infidel that I've seen before include "Non-believer, heretic" , "One who doesn't believe" , "Heathen" , "Person rejecting a specific religion" , "I don't believe" .)