Occasionally swing on ball is remarkable (6)
I believe the answer is:
signal
'remarkable' is the definition.
(signal can mean striking or noteworthy)
'occasionally swing on ball' is the wordplay.
'occasionally' indicates alternate letters.
'swing'+'on'+'ball'='swingonball'
The alternate letters of 'swingonball' are 'SIGNAL'.
'is' acts as a link.
(Other definitions for signal that I've seen before include "way to communicate" , "Remarkably good or bad; transmit (message)" , "Non-verbal action with an encoded message" , "Indicate" , "Striking; gesture" .)