Prepare to exercise leg, perhaps, and clean back (6,2)
I believe the answer is:
limber up
'prepare to exercise' is the definition.
(one may limber up before exercise)
'leg perhaps and clean back' is the wordplay.
'leg perhaps' becomes 'limb' (leg is a kind of limb).
'and' means one lot of letters go next to another.
'clean' becomes 'pure' (similar in meaning).
'back' says the letters should be written in reverse.
'pure' in reverse letter order is 'erup'.
'limb'+'erup'='LIMBER UP'
(Other definitions for limber up that I've seen before include "Exercise gently before strenuous activity" , "Get ready for exercise" , "Exercise gently prior to much greater exertion" , "Train" .)