Good film with great start (3,8)
I believe the answer is:
get cracking
'start' is the definition.
(to get cracking is to start or begin)
'good film with great' is the wordplay.
'good' becomes 'g' (abbreviation).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'with' is a charade indicator (letters next to each other).
'great' becomes 'cracking' (synonyms).
'g'+'et'+'cracking'='GET CRACKING'
(Other definitions for get cracking that I've seen before include "Start moving quickly" , "Act quickly and decisively" , "Begin!" , "Prepare to break from shell" , "Bestir oneself" .)