Joke about Spain (3)
I believe the answer is:
one
'joke' is the definition.
(as in 'that's a good one!')
'about spain' is the wordplay.
'about' becomes 'on' (as in a book 'on' a certain topic).
'spain' becomes 'e' (abbreviation for Espana).
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "Figure" , "Ace" , "Half of two" , "If upper-class, perhaps I" , "Individual" .)