Clue: They say London bloke often spouts a lot of hot air (6)
Answer: GEYSER
Explanation: Definition: often spouts a lot of hot air
Sounds like ('they say') GEEZER ('London bloke'). The "definition" is perhaps descriptive (but even so would apply more readily to the domestic water heater, rather than the geological vent, for which the hot water would be accompanied by steam rather than air; also the water heater is more commonly pronounced to match the bloke), and does not directly indicate a noun.