What does the cryptic crossword clue "Curious boy tracks what happened before start of film (9)" mean?

The wordplay uses an anagram. anagram of BOY (synonym="BOY") + TRACKS (synonym="TRACKS") = BACKSTORY The answer is BACKSTORY.

Curious boy tracks what happened before start of film (9)

Guardian Cryptic #29064 | 15 Across | 2023-05-08 flower