Prime minister exercises and belly's regularly trimmed (4)
I believe the answer is:
peel
'prime minister' is the definition.
(Robert Peel)
'exercises and belly's regularly trimmed' is the wordplay.
'exercises' becomes 'pe' (Physical Education class).
'and' is a charade indicator (letters next to each other).
'regularly trimmed' means one should take alternating letters.
The alternating letters of 'belly' are 'el'.
'pe'+'el'='PEEL'
(Other definitions for peel that I've seen before include "Shed skin" , "Remove skin of vegetable or fruit" , "See 4 and 22" , "Old PM" , "A PM; a fortified tower" .)