Dull London market with no room for meetings (6)
I believe the answer is:
leaden
'dull' is the definition.
(I've seen this before)
'london market with no room for meetings' is the wordplay.
'london market' becomes 'leadenhall' (Leadenhall Market in the City of London).
'with no' is a deletion indicator.
'room for meetings' becomes 'hall'.
'leadenhall' with 'hall' taken out is 'LEADEN'.
(Other definitions for leaden that I've seen before include "Slow-moving, being like heavy metal" , "Leaned over in slow, heavy way" , "Describes grey overcast skies" , "Depressing - heavy" , "Heavy - overcast" .)