One on one right? It's in the atmosphere! (3)
I believe the answer is:
air
'the atmosphere' is the definition.
(synonyms)
'one on one right? it's in' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' says to put letters next to each other.
'one' becomes 'a' (a thing is one thing).
'right?' becomes 'r' (common abbreviation).
'it's in' indicates putting letters inside.
'a' next to 'r' is 'ar'.
'i' put into 'ar' is 'AIR'.
(Other definitions for air that I've seen before include "Broadcast; express" , "Broadcast j" , "Demeanour" , "Tune - mood - atmosphere" , "Of which 78% is nitrogen" .)