The most common and most likely answer to this crossword clue is the 4 letter word EXON
E
X
O
N
Buddy explains!
The straight part
We know that "senior officer in the Guards" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case senior officer in the guards = exon.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 EXON mean?
EXON noun- An officer of the Queen's Body Guard of the Yeomen of the Guard.
- A region of a transcribed gene present in the final functional RNA molecule.