Oxygen in atmosphere, a combination of particles (4)
I believe the answer is:
atom
'a combination of particles' is the definition.
I can't tell whether this definition defines the answer.
'oxygen in atmosphere' is the wordplay.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'in' indicates putting letters inside.
'atmosphere' becomes 'atm' (synonyms).
'o' placed within 'atm' is 'ATOM'.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Neutron and proton" , "Smallest amount" , "A small but mighty particle" , "Small particle of matter" , "Nucleus surrounded" .)