Forward motion as prisoners go free (11)
I believe the answer is:
progression
'forward motion' is the definition.
('progression' can be a synonym of 'forward motion')
'prisoners go free' is the wordplay.
'free' indicates an anagram.
'prisoners'+'go'='prisonersgo'
'prisonersgo' anagrammed gives 'PROGRESSION'.
'as' acts as a link.
(Other definitions for progression that I've seen before include "(Mathematical) sequence" , "Gradual advancement" , "Gradual development" , "Forward motion" .)