Declared Englishman to be very effusive (6)
I believe the answer is:
geyser
'declared englishman' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for geyser that I've seen before include "Intermittent hot spring" , "upsurge" , "wall heater" , "Spring spouting steam and hot water" , "Spring that spouts hot water" .)