Area and volume reduced a smidgen (4)
I believe the answer is:
atom
'a smidgen' is the definition.
(thesaurus)
'area and volume reduced' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'and' says to put letters next to each other.
'volume' becomes 'tome' (I've seen this before).
'reduced' means to remove the last letter.
'tome' with its last letter taken away is 'tom'.
'a'+'tom'='ATOM'
(Other definitions for atom that I've seen before include "Particle of matter now divided" , "Very small object or amount" , "A tiny particle of matter with a big bang" , "Minute particle" , "Tiny particle, causes mighty explosion" .)