Reserve heads ball on one knee (4)
I believe the answer is:
book
'reserve' is the definition.
(booking is a kind of reserving)
'heads ball on one knee' is the wordplay.
'heads' indicates taking the first letters.
The first letters of 'ball on one knee' is 'BOOK'.
(Other definitions for book that I've seen before include "Reserve - bets taken on a race" , "Reserve - volume" , "Reserve; tome" , "Make a reservation" , "Leaves, bound" .)