The most common and most likely answer to this crossword clue is the 1 plus 3 plus 2 plus 4 plus 2 plus 7 letter word I VOW TO THEE MY COUNTRY
I've seen this crossword clue in The Times
I
V
O
W
T
O
T
H
E
E
M
Y
C
O
U
N
T
R
Y
Buddy explains!
The straight part
We know that "hymn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hymn = i vow to thee my country.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.