The most common and most likely answer to this crossword clue is the 6 letter word WHORES
I've seen this crossword clue in The Guardian
W
H
O
R
E
S
Buddy explains!

The straight part
We know that "Old pros" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old pros = whores.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 WHORES mean?
WHORES noun- Any person (especially a woman) who has sexual intercourse or engages in other sexual activity for payment, especially as a means of livelihood.
- A person who does, or offers to do, a demeaning or dishonourable activity for money or personal gain; someone who acts in a dishonourable way for personal advantage.
- A person who is sexually promiscuous; a slut.
- A person who is unscrupulous, especially one who compromises their principles for gain.
- A person who will violate behavioral standards to achieve something desired.
- A contemptible person.
- To prostitute oneself.
- To engage the services of a prostitute.
- To pimp; to pander.
- To promote shamelessly.
- To pursue false gods.
- To pursue false goals.
- To overuse something.