A little pasty? One must get stuck into exercises (3)
I believe the answer is:
pie
'a little pasty?' is the definition.
(pasty can mean a pie)
'one must get stuck into exercises' is the wordplay.
'one' becomes 'i' (Roman numeral).
'must get stuck into' indicates putting letters inside.
'exercises' becomes 'pe' (Physical Education class).
'i' put into 'pe' is 'PIE'.
(Other definitions for pie that I've seen before include "Pastry crust with filling" , "See 6" , "Easy thing" , "Dish baked in pastry-lined pan" , "Bird - dish - mixed type" .)