The most common and most likely answer to this crossword clue is the 4 plus 1 plus 3 letter word RENT A MOB
R
E
N
T
A
M
O
B
Buddy explains!

The straight part
We know that "Invited crowd" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case invited crowd = rent a mob.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.
What does RENT A MOB mean?
RENT A MOB noun- A group of people who can be summoned to attack a target.