The most common and most likely answer to this crossword clue is the 5 letter word SILKS
I've seen this crossword clue in The Daily Telegraph
S
I
L
K
S
Buddy explains!

The straight part
We know that "People with briefs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case people with briefs = silks.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 SILKS mean?
SILKS noun- A fine fiber excreted by the silkworm or other arthropod (such as a spider).
- A fine, soft cloth woven from silk fibers.
- Anything which resembles silk, such as the filiform styles of the female flower of maize, or the seed covering of bombaxes.
- The gown worn by a Senior (i.e. Queen's/King's) Counsel.
- A Senior (i.e. Queen's or King's) Counsel.
- (circus arts, in the plural) A pair of long silk sheets suspended in the air on which a performer performs tricks.
- (usually in the plural) The garments worn by a jockey displaying the colors of the horse's owner.
- To remove the silk from (corn).