The most common and most likely answer to this crossword clue is the 8 letter word PROMPTER
I've seen this crossword clue in The Daily Telegraph
P
R
O
M
P
T
E
R
More clues leading to the result PROMPTER
Buddy explains!
The straight part
We know that "Linesman who is ready to help a player" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case linesman who is ready to help a player = prompter.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 PROMPTER mean?
PROMPTER noun- The person who does the prompting.