This accent mark: ~ (5)
I believe the answer is:
tilde
'this accent mark' is the definition.
(type of accent used in Spanish and Portuguese)
This is all the clue.
(Other definitions for tilde that I've seen before include "Spanish cedilla" , "Squiggly mark" , "Accent (used over n) in Spanish and Portuguese" , "Style detail" , "Accent on some Spanish words" .)