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' is a reversal indicator.
'editor' becomes 'ed'.
'enters' indicates putting letters inside.
'very' becomes 'most' (I am not sure about this - if you are sure you should believe this answer much more).
'ed' in reverse letter order is 'de'.
'de' going inside 'most' is 'MODEST'.
Can you help me to learn more?
(Other definitions for modest that I've seen before include "Unostentatious" , "Humble, unpretentious" , "Demure" , "Bashful" , "Humble, meek or plain" .)