First thirteen letters? That's very small amount (4)
I believe the answer is:
atom
'very small amount' is the definition.
(I've seen this in another clue)
'first thirteen letters?' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'thirteen letters?' becomes 'tom' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'tom'='ATOM'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Not much substance" , "Particle of matter now divided" , "Basic unit of a chemical element" , "Smallest particle in an element able to take part in a chemical reaction" , "A wee bit" .)