Exercise extremes of caution eating a nut (5)
I believe the answer is:
pecan
'nut' is the definition.
(pecan is a kind of nut)
'exercise extremes of caution eating a' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'extremes of' suggests removing the centre.
'eating' indicates putting letters inside.
'caution' with its middle taken out is 'cn'.
'cn' enclosing 'a' is 'can'.
'pe'+'can'='PECAN'
(Other definitions for pecan that I've seen before include "of nuts" , "Walnut-like nut" , "Nut with sweet oily kernel" , "Pinkish-brown nut used in American pie" , "Nutty type" .)