American writer needing oxygen in gym (3)
I believe the answer is:
poe
'american writer' is the definition.
(Edgar Allan Poe)
'oxygen in gym' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'in' is an insertion indicator.
'gym' becomes 'pe' (short for Physical Education).
'o' put within 'pe' is 'POE'.
'needing' acts as a link.
(Other definitions for poe that I've seen before include "Entrance rooms; seeks support from" , "The Raven poet" , "Edgar Allan ---, poet more famous for his gothic short stories" , "American short story writer" , "Edgar Allan --, US writer" .)