Monastery earlier unknown (6)
I believe the answer is:
priory
'monastery' is the definition.
(I've seen this before)
'earlier unknown' is the wordplay.
'earlier' becomes 'prior' (synonyms).
'unknown' becomes 'y' (algebra).
'prior'+'y'='PRIORY'
(Other definitions for priory that I've seen before include "Convent subject to an abbey" , "Religious community" , "Religious residence" , "Monastery or convent" , "Abbotless monastery" .)