The most common and most likely answer to this crossword clue is the 5 letter word PROXY
I've seen this crossword clue in The Guardian
P
R
O
X
Y
More clues leading to the result PROXY
- Someone acting for another in favour of two unknowns
- Stand in for couple yet to be identified
- Substitute for axes
- Substitute for unknown quantities
- Authorised agent to make enquiries about animal
- Agent for two unknowns
- Agency putting pressure on old cinema?
- Nose absorbs source of meat substitute
- Representative for unknown figures
- Substitute for male sexual identity
Buddy explains!

The straight part
We know that "Substitute" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case substitute = proxy.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 PROXY mean?
PROXY noun- An agent or substitute authorized to act for another person.
- The authority to act for another, especially when written.
- The written appointment of a proctor in suits in the ecclesiastical courts.
- A measurement of one physical quantity that is used as an indicator of the value of another
- An interface for a service, especially for one that is remote, resource-intensive, or otherwise difficult to use directly.
- To serve as a proxy for.
- To function as a server for a client device, but pass on the requests to another server for service.
- Used as a proxy or acting as a proxy.
- A proximity mine; a mine that explodes when something approaches within a certain distance.