The most common and most likely answer to this crossword clue is the 4 letter word UMBO
I've seen this crossword clue in The Times
U
M
B
O
Buddy explains!
The straight part
We know that "boss" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boss = umbo.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 UMBO mean?
UMBO noun- A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane. For example the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.
- One of the lateral prominences just above the hinge of a bivalve shell.
- A bump or protrusion on the top of the cap.
- The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.