Able to follow exercise nut (5)
I believe the answer is:
pecan
'nut' is the definition.
(pecan is a kind of nut)
'able to follow exercise' is the wordplay.
'able' becomes 'can'.
'to follow' means one lot of letters go next to another.
'exercise' becomes 'pe' (Physical Education).
'can' after 'pe' is 'PECAN'.
(Other definitions for pecan that I've seen before include "Edible nut like a smooth walnut" , "Species of hickory - edible nut" , "Nutty type" , "Nut of American hickory tree" , "Walnut-like nut" .)