Choice area infiltrated by the Spanish (9)
I believe the answer is:
selection
'choice' is the definition.
('selection' can be a synonym of 'choice')
'area infiltrated by the spanish' is the wordplay.
'area' becomes 'section' (section is a kind of area**).
'infiltrated by' is an insertion indicator.
'the spanish' becomes 'el'.
'section' going around 'el' is 'SELECTION'.
(Other definitions for selection that I've seen before include "Act of choosing" , "Horse tipped for betting" , "Choice, pick" .)