Less shallow (6)
I believe the answer is:
deeper
'less shallow' is the definition.
('deep' can be the opposite of 'shallow' so perhaps 'deeper' is reasonable for 'less shallow')
This is all the clue.
(Other definitions for deeper that I've seen before include "Profounder" , "Lower in pitch" , "More profound or more intense" , "Wiser" , "more involved" .)