Form of rummy (7)
I believe the answer is:
'form of rummy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for canasta that I've seen before include "this needs two packs" , "Game played with two packs of cards and jokers" , "Two-pack card game" , "Card-game played with two packs of cards" , "Rummy-like card-game" .)