Quote line about a Duke in fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(thesaurus)
'quote line about a duke' is the wordplay.
'quote' becomes 'cite' ('cite' can be a synonym of 'quote'**).
'line' becomes 'l'.
'about' indicates putting letters inside.
'duke' becomes 'd'.
'cite'+'l'='citel'
'a'+'d'='ad'
'citel' placed around 'ad' is 'CITADEL'.
'in' acts as a link.
(Other definitions for citadel that I've seen before include "secure place" , "place of refuge" , "Urban fortress" , "Urban stronghold - dialect (anag)" , "Stronghold (of the Salvation Army?)" .)