Pervert Frenchman's quick to seize Spanish aunt (7)
I believe the answer is:
vitiate
'pervert' is the definition.
(both can mean to spoil or mar)
'frenchman's quick to seize spanish aunt' is the wordplay.
'frenchman's quick' becomes 'vite' (I can't justify this - if you can you should believe this answer much more).
'to seize' means one lot of letters goes inside another.
'spanish aunt' becomes 'tia' ('aunt' in Spanish).
'vite' placed around 'tia' is 'VITIATE'.
Can you help me to learn more?
(Other definitions for vitiate that I've seen before include "Debase - render defective" , "ruin" , "Corrupt" .)