Key discussion curtailed after six (5)
I believe the answer is:
vital
'key' is the definition.
(thesaurus)
'discussion curtailed after six' is the wordplay.
'discussion' becomes 'talk' (talk is a kind of discussion).
'curtailed' means to remove the last letter.
'after' means one lot of letters go next to another.
'six' becomes 'vi' (roman numeral).
'talk' with its last letter removed is 'tal'.
'tal' after 'vi' is 'VITAL'.
(Other definitions for vital that I've seen before include "Of the greatest importance" , "Characteristic of life" , "Crucially important, essential" , "Highly important" , "Absolutely necessary or life-giving" .)