Low in pitch (4)
(Other definitions for bass that I've seen before include "Lowest register of the male voice" , "bottom of sound?" , "Fish for the low-voiced" , "Very low - perch" , "Fish (which sings?)" .)
'low in pitch' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for deep that I've seen before include "greatly involved?" , "Sunk low" , "profoundly" , "Cavernous" , "Intense; low (in pitch)" .)