The most common and most likely answer to this crossword clue is the 15 letter word THEÂ LONGESTÂ DAY
I've seen this crossword clue in The Guardian
T
H
E
Â
L
O
N
G
E
S
T
Â
D
A
Y
Buddy explains!
The straight part
We know that "Film made about 6/21 June" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case film made about 6/21 june = the longest day.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.