It's a disappointment, being allowed soft feathers (3,4)
I believe the answer is:
let down
'it's a disappointment' is the definition.
(I've seen this before)
'allowed soft feathers' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'soft feathers' becomes 'down' (I've seen this in another clue).
'let'+'down'='LET-DOWN'
'being' acts as a link.
(Other definitions for let down that I've seen before include "blow" , "Disappoint or fail - or lengthen garment" , "Disappoint, fail to support" , "deflating" , "Disappoint, fail to keep promise" .)