Six having whip-round that's generous (6)
I believe the answer is:
lavish
'generous' is the definition.
(similar in meaning)
'six having whip-round' is the wordplay.
I cannot really understand how this works, but
'six' could be 'vi' (Roman numerals) and 'vi' is located in the answer.
'whip' could be 'lash' (synonyms) and 'lash' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for lavish that I've seen before include "Heap (money)" , "Spend(ing) generously" , "Plentiful or munificent" , "Sumptuous" , "Plentiful, in large quantities like display" .)