Essential — lively (5)
I believe the answer is:
vital
This clue is a double definition.
'essential' is the first definition.
(similar in meaning)
'lively' is the second definition.
(I know that lively can be written as vital)
(Other definitions for vital that I've seen before include "Extremely important, essential to success" , "Desperately important" , "Of the greatest importance" , "Critical; life-sustaining" , "Spirited" .)