They say London bloke often spouts a lot of hot air (6)
I believe the answer is:
geyser
'often spouts a lot of hot air' is the definition.
(geographical feature which spouts air or water)
'they say london bloke' is the wordplay.
'they say' indicates a 'sounds like' (homophone) clue.
'london bloke' becomes 'geezer' (London term for a man).
'geezer' sounds like 'GEYSER'.
(Other definitions for geyser that I've seen before include "It gushes steam and hot water" , "Gas-fired water heater" , "Spring that discharges steam and hot water" , "wall heater" , "Steaming spring" .)