The most common and most likely answer to this crossword clue is the 5 letter word LOCUM
I've seen this crossword clue in The Guardian
L
O
C
U
M
More clues leading to the result LOCUM
- Doctor and I don't know what to say after colonel turns up
- Sub's back pass with sign of uncertainty
- Sub officer retreating? I'm not sure
- Unfamiliar doctor having short hair? I'm not sure
- Powerless mass accepting military leader’s stand-in
- Deputy‘s also scrummy occasionally
- Substitute ballplayers for pacey but outnumbered centres
- Nothing in powder, no thanks — here’s a medical replacement
- One’s arrival is a relief for the surgery
- Stand in line backing company’s union man initially
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 = locum.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 LOCUM mean?
LOCUM noun- Short for locum tenens.
- A share in a Genoese trading vessel.