Speech one’s rewritten — about Royal Marines (6)
I believe the answer is:
sermon
'speech' is the definition.
(I know that sermon is a type of speech)
'one's rewritten about royal marines' is the wordplay.
'rewritten' is an anagram indicator.
'about' indicates putting letters inside.
'royal marines' becomes 'RM' (abbreviation).
'ones' is an anagram of 'seon'.
'seon' going around 'rm' is 'SERMON'.
(Other definitions for sermon that I've seen before include "Lesson or religious talk" , "Address given weekly" , "minister's speech" , "Moralising lecture" , "Pulpit speech" .)