The most common and most likely answer to this crossword clue is the 7 letter word RELIEFS
I've seen this crossword clue in The Daily Telegraph
R
E
L
I
E
F
S
More clues leading to the result RELIEFS
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 = reliefs.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 RELIEFS mean?
RELIEFS noun- The removal of stress or discomfort.
- The feeling associated with the removal of stress or discomfort.
- Release from a post or duty, as when replaced by another.
- The person who takes over a shift for another.
- Aid or assistance offered in time of need.
- Court-ordered compensation, aid, or protection, a redress.
- A lowering of a tax through special provisions; tax relief.
- A certain fine or composition paid by the heir of a tenant upon the death of the ancestor.
- A type of sculpture or other artwork in which shapes or figures protrude from a flat background.
- The apparent difference in elevation in the surface of a painting or drawing made noticeable by a variation in light or color.
- The difference of elevations on a surface.
- The supposed projection of a charge from the surface of a field, indicated by shading on the sinister and lower sides.