Before mounting musical, director finally ran over story (9)
I believe the answer is:
narrative
'story' is the definition.
(I know that story can be written as narrative)
'before mounting musical director finally ran over' is the wordplay.
'before' means one lot of letters go next to another.
'mounting' says the letters should be written backwards (going up in a down clue).
'musical' becomes 'evita' (Andrew Lloyd Webber musical).
'finally' indicates one should take the final letters.
'over' shows that the letters should be reversed in order.
The last letter of 'director' is 'r'.
'evita' written backwards gives 'ative'.
'r'+'ran'='rran'
'rran' in reverse letter order is 'narr'.
'ative' put after 'narr' is 'NARRATIVE'.
(Other definitions for narrative that I've seen before include "Report" , "Story line" , "Spoken account; story" , "Verbal account" , "Storyline" .)