Relatively speaking the greater part of an eruption (5)
I believe the answer is:
uncle
'eruption' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for uncle that I've seen before include "Family relation" , "Male family member" , "Slang name for pawnbroker" , "Pawnbroker's nickname" , "one who's taken the pledge" .)