Spencer changing amateur film script (10)
I believe the answer is:
'film script' is the definition.
(I've seen this before)
'spencer changing amateur' is the wordplay.
'changing' indicates an anagram.
'amateur' becomes 'lay' (similar in meaning).
'spencer' with letters rearranged gives 'screenp'.
(Other definitions for screenplay that I've seen before include "Scenario" , "Script of film with directions" , "full instructions for shooting here" , "part of a film" , "Instructions to actors etc" .)