Cast off when one's covered in feathers (6)
I believe the answer is:
disown
'cast off' is the definition.
(I've seen this in another clue)
'one's covered in feathers' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'covered in' indicates putting letters inside.
'feathers' becomes 'down' (down can mean bird feathers).
'is' placed into 'down' is 'DISOWN'.
'when' acts as a link.
(Other definitions for disown that I've seen before include "Disinherit or renounce" , "Disclaim all connection or relationship with" , "Forsake" , "Repudiate or refuse to acknowledge a relationship" , "Reject all connection with, refuse to acknowledge" .)