The most common and most likely answer to this crossword clue is the 3 plus 2 plus 3 plus 4 letter word PUT ON ONE SIDE
I've seen this crossword clue in The Guardian
P
U
T
O
N
O
N
E
S
I
D
E
More clues leading to the result PUT ON ONE SIDE
Buddy explains!
The straight part
We know that "reserve" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reserve = put on one side.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.