Prince William finally gets a game (5)
I believe the answer is:
'game' is the definition.
'prince william finally gets a' is the wordplay.
'prince' becomes 'hal' (Shakespearean nickname of Henry V when a prince).
'finally' says to take the final letters.
'gets' is a charade indicator (letters next to each other).
The last letter of 'william' is 'm'.
(Other definitions for halma that I've seen before include "played on board" , "Hopping board game" , "Leaping board game" , "Old board game" , "Board-game" .)