The most common and most likely answer to this crossword clue is the 4 letter word VETS
I've seen this crossword clue in The Daily Telegraph
V
E
T
S
Buddy explains!
The straight part
We know that "Runs security check on/retired servicemen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case runs security check on/retired servicemen = vets.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 VETS mean?
VETS noun- A veterinarian or veterinary surgeon.
- A veteran (a former soldier or other member of an armed forces).
- To thoroughly check or investigate particularly with regard to providing formal approval.