Key alien's set free (9)

I believe the answer is:

essential

'key' is the definition.
(thesaurus)

'alien's set free' is the wordplay.
'free' is an anagram indicator.
'aliens'+'set'='aliensset'
'aliensset' with letters rearranged gives 'ESSENTIAL'.

(Other definitions for essential that I've seen before include "organic" , "Indispensible" , "Basic or necessary" , "Vitally important" , "Key" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.