Exercise hard with a cheek raised (7)
I believe the answer is:
pilates
'exercise' is the definition.
(pilates is a type of exercise)
'hard with a cheek raised' is the wordplay.
'hard' becomes 'set' (similar in meaning).
'with' says to put letters next to each other.
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'raised' shows that the letters should be reversed in order (going up in a down clue).
'set'+'a'+'lip'='setalip'
'setalip' back-to-front is 'PILATES'.
(Other definitions for pilates that I've seen before include "Exercise system" , "attributed to Biblical procurator?" , "Ali's pet (anag) - exercise regime" , "Holistic system of physical exercises" , "Set of exercises used in holistic medicine" .)