Player is entering races on ice (10)

'player' is the definition.
(I know that cornettist is a type of player)

'is entering races on ice' is the wordplay.
'entering' indicates putting letters inside.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'on' says to put letters next to each other.
'ice' becomes 'cornet' (type of ice cream).
'is' inserted inside 'tt' is 'tist'.
'tist' put after 'cornet' is 'CORNETTIST'.