Conservative series of lectures in large hall (9)
I believe the answer is:
concourse
'large hall' is the definition.
'concourse' can be an answer for 'hall' (concourse is a kind of hall). I am not sure about the 'large' bit.
'conservative series of lectures' is the wordplay.
'conservative' becomes 'con' (abbreviation for Conservative).
'series of lectures' becomes 'course' (I've seen this before).
'con'+'course'='CONCOURSE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for concourse that I've seen before include "spacious area?" , "Large open space in, eg, railway station" , "A gathering of people" , "crowd" , "Crowds" .)