Duck can maintain feathers (4,4)
I believe the answer is:
keep down
'duck' is the definition.
(I've seen this before)
'maintain feathers' is the wordplay.
'maintain' becomes 'keep' (synonyms).
'feathers' becomes 'down' (down can mean bird feathers).
'keep'+'down'='KEEP DOWN'
'can' acts as a link.
(Other definitions for keep down that I've seen before include "Repress - lie low" , "Remain out of sight" , "Duck" , "Don't be seen" .)