Different? Spain’s the same on the outside (4)
I believe the answer is:
else
'different?' is the definition.
('else' can be a synonym of 'different')
'spain's the same on the outside' is the wordplay.
'spain's the' becomes 'el' ('the' in Spanish).
'on the outside' means to remove the middle letters.
'same' with its middle taken out is 'se'.
'el'+'se'='ELSE'
(Other definitions for else that I've seen before include "Alternatively" , "Otherwise; different from" , "Besides, otherwise" , "Other than (what is under consideration)" , "As an alternative" .)