Exercise is turned, put back (3-2)
I believe the answer is:
sit-up
'exercise' is the definition.
(I know that sit-up is a type of stomach exercise)
'is turned put back' is the wordplay.
'turned' is a reversal indicator.
'back' says the letters should be written in reverse.
'is' written backwards gives 'si'.
'put' back-to-front is 'tup'.
'si'+'tup'='SIT-UP'
(Other definitions for sit-up that I've seen before include "Exercise done on the floor" , "Exercise designed to strengthen stomach muscles" , "Physical exercise" .)