Superficial, lacking depth (7)
I believe the answer is:
shallow
I believe this clue is a double definition.
'superficial' is the first definition.
(similar in meaning)
'lacking depth' is the second definition.
(I've seen this before)
(Other definitions for shallow that I've seen before include "flimsy" , "Without depth" , "Trivial" , "Of water, not deep" , "Superficial" .)