Key turned as let in after entrances cleared (9)
I believe the answer is:
essential
'key' is the definition.
(I've seen this before)
'turned as let in after entrances cleared' is the wordplay.
'turned' is an anagram indicator.
'after' means one lot of letters go next to another.
'cleared' suggests removing the centre (the inside letters are cleared out).
'entrances' with its middle removed is 'es'.
'as'+'let'+'in'='asletin'
'asletin' is an anagram of 'sential'.
'sential' after 'es' is 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "as oil might be" , "Key" , "So is certain oil-production" , "Indispensible" , "'Absolutely necessary, required (9)'" .)