Prosper — not, say, going backwards (3,2)
I believe the answer is:
get on
'prosper' is the definition.
(I've seen this before)
'not say going backwards' is the wordplay.
'say' becomes 'eg' ().
'going backwards' is a reversal indicator.
'not'+'eg'='noteg'
'noteg' written backwards gives 'GET ON'.
(Other definitions for get on that I've seen before include "Make progress or climb aboard" , "Board - prosper" , "Obtain promotion" , "Board (a bus)" , "Enjoy harmonious relationship" .)