Some Haagen-Dazs? That's a plan! (6)
I believe the answer is:
agenda
'a plan' is the definition.
(I know that agenda is a type of plan)
'some haagen-dazs? that's' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'AGENDA' is hidden within 'haagen dazs that'.
(Other definitions for agenda that I've seen before include "Meet to discuss this" , "Items of business for committee meeting" , "Programme of business" , "List of matters to be discussed at a meeting" , "Topic list for meeting" .)