The most common and most likely answer to this crossword clue is the 7 letter word PROXIES
I've seen this crossword clue in The Daily Telegraph
P
R
O
X
I
E
S
Buddy explains!
The straight part
We know that "Substitutes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case substitutes = proxies.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 PROXIES mean?
PROXIES 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.
- A proximity mine; a mine that explodes when something approaches within a certain distance.