The retiring editor enters, very unassuming (6)
I believe the answer is:
modest
'unassuming' is the definition.
(similar in meaning)
'the retiring editor enters very' is the wordplay.
'the retiring' says the letters should be written backwards.
'editor' becomes 'ed' (abbreviation).
'enters' indicates putting letters inside.
'very' becomes 'most' (I've seen this in another clue).
'ed' written backwards gives 'de'.
'de' placed into 'most' is 'MODEST'.
(Other definitions for modest that I've seen before include "Unostentatious" , "Humble, unpretentious" , "Demure" , "Bashful" , "Humble, meek or plain" .)