Illustrations involving one system of exercise (7)
I believe the answer is:
pilates
'system of exercise' is the definition.
(I've seen this before)
'illustrations involving one' is the wordplay.
'illustrations' becomes 'plates' (plate is a kind of illustration).
'involving' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'plates' going around 'i' is 'PILATES'.
(Other definitions for pilates that I've seen before include "Modern exercise" , "Exercise system" , "Pleats I arrange into a form of exercise" , "Ali's pet (anag) - exercise regime" , "Holistic system of physical exercises" .)