A bride's unhappy with such a mattress (3,3)
I believe the answer is:
air bed
'such a mattress' is the definition.
(air-bed is a kind of mattress)
'a bride's unhappy' is the wordplay.
'unhappy' indicates anagramming the letters.
'bride' is an anagram of 'irbed'.
'a'+'irbed'='AIR-BED'
'with' acts as a link.
(Other definitions for air bed that I've seen before include "place to sleep" , "Inflatable mattress" .)