Weak, insubstantial (6)
I believe the answer is:
flimsy
This clue is a double definition.
'weak' is the first definition.
(similar in meaning)
'insubstantial' is the second definition.
(I know that insubstantial can be written as flimsy)
(Other definitions for flimsy that I've seen before include "Insubstantial, light and thin" , "Fragile; weak" , "Light and thin, easily torn or broken" , "Lightweight" , "Light and thin, material or argument, perhaps" .)