A certain amount of passion, but not much substance (3)
I believe the answer is:
ion
'substance' is the definition.
(ion is a kind of substance)
'a certain amount of passion but not much' is the wordplay.
'ION' is hidden in amongst the letters.
However, I am unsure how this is indicated.
Can you help me to learn more?
(Other definitions for ion that I've seen before include "Electrically charged particle" , "Eletrically charged particle" , "Electrically-charged particle" , "Subatomic particle" , "Particle (phys.)" .)