The most common and most likely answer to this crossword clue is the 7 letter word ENGAGES
I've seen this crossword clue in The Daily Telegraph
E
N
G
A
G
E
S
More clues leading to the result ENGAGES
Buddy explains!

The straight part
We know that "reserves" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reserves = engages.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 ENGAGES mean?
ENGAGES verb- (heading) To interact socially.
- (heading) To interact antagonistically.
- (heading) To interact contractually.
- (heading) To interact mechanically.
- To enter into (an activity), to participate (construed with in).
- To entangle.