Chivalrous in speech behind castle at regular intervals? (11)
I believe the answer is:
fortnightly
'at regular intervals?' is the definition.
(I've seen this before)
'chivalrous in speech behind castle' is the wordplay.
'chivalrous' becomes 'knightly' (like a knight).
'in speech' shows a homophone (sound like).
'behind' is a charade indicator (letters next to each other).
'castle' becomes 'fort' (fort is a type of castle).
'knightly' is a homophone of 'nightly'.
'nightly' after 'fort' is 'FORTNIGHTLY'.
(Other definitions for fortnightly that I've seen before include "at regular intervals" , "Every two weeks" , "Every fourteen days" .)