Not much material in first thirteen letters? (4)
I believe the answer is:
atom
'not much material' is the definition.
'first thirteen letters?' is the wordplay.
'first' becomes 'a'.
'thirteen letters?' becomes 'tom' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'tom'='ATOM'
'in' is the link.
(Other definitions for atom that I've seen before include "An explosive particle as in bomb" , "A mighty physical particle or bomb" , "Very small bit" , "A mighty particle, makes huge explosion" , "Nucleus plus electrons" .)