Writer Oscar stops exercises (3)
I believe the answer is:
poe
'writer' is the definition.
(Edgar Allan Poe)
'oscar stops exercises' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'stops' is an insertion indicator.
'exercises' becomes 'pe' (Physical Education class).
'o' inserted inside 'pe' is 'POE'.
(Other definitions for poe that I've seen before include "Entrance rooms; seeks support from" , "US horror master" , "Edgar Allan ..., American writer of the macabre" , "Edgar Allen, great American writer of eerie stories" , "Author of The Murders in the Rue Morgue" .)