In fact in favour of Otho's replacement (8)
I believe the answer is:
forsooth
'in fact' is the definition.
(thesaurus)
'in favour of otho's replacement' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'replacement' indicates an anagram.
'othos' with letters rearranged gives 'sooth'.
'for'+'sooth'='FORSOOTH'
(Other definitions for forsooth that I've seen before include "truly" , "Marry" , "In truth (arch)" , "certainly" , "Indeed (archaic)" .)