Writing on reflection remains a great success (5)
I believe the answer is:
smash
'a great success' is the definition.
(thesaurus)
'writing on reflection remains' is the wordplay.
'writing' becomes 'MS' (abbreviation for manuscript).
'on reflection' says the letters should be written backwards.
'remains' becomes 'ash' (remains of a fire).
'ms' backwards is 'sm'.
'sm'+'ash'='SMASH'
(Other definitions for smash that I've seen before include "- - and grab" , "Violently break into pieces" , "something very popular" , "tremendous success!" , "Break suddenly into little pieces" .)