The most common and most likely answer to this crossword clue is the 6 plus 3 letter word MASTER KEY
I've seen this crossword clue in The Guardian
M
A
S
T
E
R
K
E
Y
More clues leading to the result MASTER KEY
Buddy explains!
The straight part
We know that "required for Common Entrance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case required for common entrance = master key.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.