The most common and most likely answer to this crossword clue is the 9 letter word MECHANICS
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More clues leading to the result MECHANICS
Buddy explains!

The straight part
We know that "men who perform service" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case men who perform service = mechanics.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 MECHANICS mean?
MECHANICS noun- The branch of physics that deals with the action of forces on material objects with mass
- The design and construction of machines.
- (writing) Spelling and punctuation.
- Operation in general; workings.
- A manual worker; a labourer or artisan.
- Someone who builds or repairs machinery, a technician; now specifically, someone who works with and repairs the mechanical parts of a motor vehicle, aircraft or similar.
- A device, command, or feature which allows someone to achieve a specific task.
- A hit man.