Lacking substance; narrow (4)
I believe the answer is:
thin
I believe this is a double definition.
'lacking substance' is the first definition.
(I've seen this in another clue)
'narrow' is the second definition.
(associated in meaning)
(Other definitions for thin that I've seen before include "Sparse; not dense" , "Superficial" , "Implausible" , "as 7 [LOAF] may be sliced" , "Made of light material" .)