Prior to haunt illuminated cloisters (5)
I believe the answer is:
until
'prior to' is the definition.
(I've seen this in another clue)
'haunt illuminated cloisters' is the wordplay.
'UNTIL' is hidden within the letters.
However, I am uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for until that I've seen before include "Before a given time" , "Up to that moment" , "Unlit (anag.)" , "As late as" , "Pending such time as" .)