The most common and most likely answer to this crossword clue is the 11 letter word FLAG-WAVING
I've seen this crossword clue in The Times
F
L
A
G
-
W
A
V
I
N
G
Buddy explains!
The straight part
We know that "declaring his allegiance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case declaring his allegiance = flag-waving.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.