Book a player unlikely to play (7)
I believe the answer is:
reserve
'book a player' is the definition.
'reserve' can be an answer for 'book' (as in booking or reserving in advance). I am not certain of the 'a player' bit.
'unlikely to play' is the wordplay.
'unlikely' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'to play' becomes 'serve' (serve is a kind of play).
're'+'serve'='RESERVE'
Can you help me to learn more?
(Other definitions for reserve that I've seen before include "Kept back stock" , "Keep for someone - spare bod" , "Set aside - shyness" , "some distance" , "Book - substitute player" .)