The most common and most likely answer to this crossword clue is the 4 plus 9 letter word RULE BRITANNIA
R
U
L
E
B
R
I
T
A
N
N
I
A
More clues leading to the result RULE BRITANNIA
Buddy explains!
The straight part
We know that "patriotic song" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case patriotic song = rule britannia.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.