Hotel annex in which spectre finally materialised (8)
I believe the answer is:
happened
'materialised' is the definition.
('happen' can be a synonym of 'materialize')
'hotel annex in which spectre finally' is the wordplay.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'annex' becomes 'append' (I've seen this in another clue).
'in which' indicates putting letters inside.
'finally' says to take the final letters.
The last letter of 'spectre' is 'e'.
'append' enclosing 'e' is 'appened'.
'h'+'appened'='HAPPENED'
(Other definitions for happened that I've seen before include "Took place" , "Occurred" , "Came to pass" .)