Bulb very bright at last across room (7)
I believe the answer is:
shallot
'bulb' is the definition.
(shallot is a kind of bulb)
'very bright at last across room' is the wordplay.
'very bright at last' becomes 'sot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'across' is an insertion indicator.
'room' becomes 'hall' (hall is a kind of room).
'sot' going around 'hall' is 'SHALLOT'.
Can you help me to learn more?
(Other definitions for shallot that I've seen before include "Species of onion" , "Small onion" , "Edible onion-like bulb" , "Type of onion" , "Small onion-like bulb used for seasoning" .)