The most common and most likely answer to this crossword clue is the 10 letter word UNDERSTUDY
I've seen this crossword clue in The Times
U
N
D
E
R
S
T
U
D
Y
More clues leading to the result UNDERSTUDY
- Do too little work and prepare to play, given the chance
- Reserve covered by boss over conclusion of enquiry
- Waiter offstage being pinned down by Casanova, ending in hilarity
- One acts in emergency ward, finally, with duty nurse needing replacement
- Died: duty nurse being replaced by junior
- Fallback option being given close consideration
- Don’t read enough to be a locum?
- Actor is subject to scrutiny
- Reserve player
- Deputy hypnotised boss by the end of the day
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 = understudy.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 UNDERSTUDY mean?
UNDERSTUDY noun- A performer who understudies; a standby.
- To study or know a role to such an extent as to be able to replace the normal performer when required.
- To act as an understudy (to someone).
- To act in a similar manner to some known person.