The Persians' outlandish figure of speech (11)
I believe the answer is:
parenthesis
'figure of speech' is the definition.
(thesaurus)
'the persians' outlandish' is the wordplay.
'outlandish' indicates anagramming the letters.
'the'+'persians'='thepersians'
'thepersians' anagrammed gives 'PARENTHESIS'.
(Other definitions for parenthesis that I've seen before include "something added in text" , "phrase by way of explanation" , "Words contained" , "Bracket" , "interval" .)