A little baby's sallow from a very deep bed (7)
I believe the answer is:
abyssal
'from a very deep bed' is the definition.
(abyssal can mean relating to the sea bed)
'a little baby's sallow' is the wordplay.
'a little' indicates a hidden word.
'ABYSSAL' is hidden in the letters of 'babys sallow'.
(Other definitions for abyssal that I've seen before include "Bottomless" , "regarding ocean trenches?" , "on the bed" , "Belonging to ocean depths" .)