Hearts extremely optimistic ahead of crucial game (6)
I believe the answer is:
hockey
'game' is the definition.
(hockey is a kind of game)
'hearts extremely optimistic ahead of crucial' is the wordplay.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'extremely' says to hollow out the word (remove centre letters).
'ahead of' says to put letters next to each other (I've seen this in other clues).
'crucial' becomes 'key' (I've seen this before).
'optimistic' with its centre removed is 'oc'.
'h'+'oc'+'key'='HOCKEY'
(Other definitions for hockey that I've seen before include "Ball-game" , "Stick and ball game" , "SPORT/GAME" , "It's played with curved stick" , "Team game using curved sticks and ball" .)