The most common and most likely answer to this crossword clue is the 13 letter word UNDERSTUDYING
I've seen this crossword clue in The Sunday Telegraph
U
N
D
E
R
S
T
U
D
Y
I
N
G
Buddy explains!
The straight part
We know that "Preparing to substitute for player" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preparing to substitute for player = understudying.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 UNDERSTUDYING mean?
UNDERSTUDYING verb- 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.