Illustrations describing one exercise programme (7)
I believe the answer is:
pilates
'exercise programme' is the definition.
(type of exercise programme)
'illustrations describing one' is the wordplay.
'illustrations' becomes 'plates' (plate is a kind of illustration).
'describing' is an insertion indicator (describe can mean to form the outline of).
'one' becomes 'i' (Roman numeral).
'plates' going around 'i' is 'PILATES'.
(Other definitions for pilates that I've seen before include "Pleats I arrange into a form of exercise" , "regime for improved fitness" , "Exercise system" , "Set of exercises used in holistic medicine" , "Modern exercise" .)