I sent Les a replacement key (9)

I believe the answer is:

essential

'key' is the definition.
(I've seen this before)

'i sent les a replacement' is the wordplay.
'replacement' indicates an anagram.
'i'+'sent'+'les'+'a'='isentlesa'
'isentlesa' anagrammed gives 'ESSENTIAL'.

(Other definitions for essential that I've seen before include "Vital" , "Indispensible" , "Basic or necessary" , "absolutely neccessary?" , "Crucial" .)

I've seen this clue in The Globe and Mail.

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.