Article on Spain for you once (4)
I believe the answer is:
thee
'you once' is the definition.
(historical second person pronoun)
'article on spain' is the wordplay.
'article' becomes 'the' (the definite article).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'spain' becomes 'e' (abbreviation for Espana).
'the'+'e'='THEE'
'for' is the link.
(Other definitions for thee that I've seen before include "'U'" , "you many years ago" , "Poetically you" , "Old objective second person singular" , "You in the olden days" .)