Exercise hard, a cheek aloft (7)
I believe the answer is:
pilates
'exercise' is the definition.
(pilates is a type of exercise)
'hard a cheek aloft' is the wordplay.
'hard' becomes 'set' ('set' can be similar in meaning to 'hard'**).
'cheek' becomes 'lip' (lip is a kind of cheek).
'aloft' shows that the letters should be reversed in order.
'set'+'a'+'lip'='setalip'
'setalip' written backwards gives 'PILATES'.
(Other definitions for pilates that I've seen before include "Ali's pet (anag) - exercise regime" , "regime for improved fitness" , "Holistic system of physical exercises" , "attributed to Biblical procurator?" , "Modern exercise" .)