American nut exercises about noon (5)
I believe the answer is:
pecan
'american nut' is the definition.
(type of nut)
'exercises about noon' is the wordplay.
'exercises' becomes 'pe' (Physical Education class).
'about' becomes 'ca' (short for circa).
'noon' becomes 'n'.
'pe'+'ca'+'n'='PECAN'
(Other definitions for pecan that I've seen before include "Nut of American hickory tree" , "Nut with sweet oily kernel" , "Species of hickory - edible nut" , "Type of nut" , "Walnut-like nut" .)