Deceive a dandy about the Spanish (6)
I believe the answer is:
delude
'deceive' is the definition.
('delude' can be a synonym of 'deceive')
'a dandy about the spanish' is the wordplay.
'a dandy' becomes 'dude' ('dude' can be a synonym of 'dandy').
'about' is an insertion indicator.
'the spanish' becomes 'el'.
'dude' going around 'el' is 'DELUDE'.
(Other definitions for delude that I've seen before include "Mislead or fool" , "Mislead in belief" , "Mislead; deceive" , "Cause to believe something untrue, deceive" , "swindle" .)