Imperative (9)
'imperative' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for essential that I've seen before include "something necessary" , "'Absolutely necessary, required (9)'" , "Type of oil" , "absolutely neccessary?" , "Vitally important" .)
'imperative' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vital that I've seen before include "Most important, like certain body organs" , "Essential to life, indispensable" , "Imperative" , "Crucially important, essential" , "Urgently needed or absolutely necessary" .)