Scrap the first thirteen letters (4)
I believe the answer is:
atom
'scrap' is the definition.
(thesaurus)
'the first thirteen letters' is the wordplay.
'the first' becomes 'a'.
'thirteen letters' becomes 'tom' (I can't explain this - if you can you should believe this answer much more).
'a'+'tom'='ATOM'
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Tiny particle, great bomb" , "A mighty particle, makes huge explosion" , "Nucleus surrounded" , "Nucleus plus electrons" , "A mighty particle, can be explosive" .)