Beam crossing room in tavern, initially a curiosity (6)
I believe the answer is:
rarity
'a curiosity' is the definition.
('rarity' can be a synonym of 'curiosity')
'beam crossing room in tavern initially' is the wordplay.
'beam' becomes 'ray' ('ray' can be a synonym of 'beam').
'crossing' indicates putting letters inside.
'initially' says to take the initial letters.
The initial letters of 'room in tavern' is 'rit'.
'ray' placed around 'rit' is 'RARITY'.
(Other definitions for rarity that I've seen before include "Unusual thing" , "Uncommon thing" , "'Something unusual, perhaps collectable (6)'" , "Something very scarce and often valuable" , "Very unusual occurrence" .)