Gambling occasionally on fall of cards? It’s of some moment (1,3,4)

'it's of some moment' is the definition.
(something momentous)

'gambling occasionally on fall of cards?' is the wordplay.
'occasionally' indicates alternate letters.
'on' is a charade indicator (letters next to each other).
'fall of cards?' becomes 'deal' (how cards happen to have been dealt).
The alternate letters of 'gambling' are 'abig'.
'abig'+'deal'='A BIG DEAL'