Writer protected by editor when in depression, as a hanger-on (9)
I believe the answer is:
dependent
'a hanger-on' is the definition.
(thesaurus)
'writer protected by editor when in depression' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'protected by' is an insertion indicator.
'editor' becomes 'ed'.
'when in' means one lot of letters goes inside another.
'depression' becomes 'dent' ('dent' can be a synonym of 'depression'**).
'pen' going inside 'ed' is 'epend'.
'epend' put into 'dent' is 'DEPENDENT'.
'as' acts as a link.
(Other definitions for dependent that I've seen before include "Reliant" , "Subject" , "Relying/contingent (on)" .)