A little baby's sallow from a very deep bed (7)
I believe the answer is:
'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 "on the bed", "regarding ocean trenches?", "Belonging to ocean depths", "Bottomless".)