Theme tune of originally short film (5)
I believe the answer is:
topic
'theme' is the definition.
(I know that theme can be written as topic)
'tune of originally short film' is the wordplay.
'originally' suggests taking the first letters.
'short film' becomes 'pic' (short for picture; 'short' asks for a short form).
The first letters of 'tune of' is 'to'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Content of argument" , "Subject matter, theme" , "Theme of discourse" , "A subject for conversation or debate" , "Subject (eg, of text)" .)