Start to play badly with the ball (4)
I believe the answer is:
pill
'the ball' is the definition.
(humorous term for the ball in some sports)
'start to play badly' is the wordplay.
'start to' says to take the initial letters.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
The initial letter of 'play' is 'p'.
'p'+'ill'='PILL'
'with' acts as a link.
(Other definitions for pill that I've seen before include "Medicinal pellet" , "nuisance to put up with" , "Medical tablet" , "Medicinal tablet" , "may be the cure" .)