It's bulbous and down to earth (4)
I believe the answer is:
corm
'earth' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for corm that I've seen before include "Underground feature of crocus, gladiolus etc." , "Underground part of, eg, cyclamen" , "Underground bulb-shaped stem" , "Underground part of iris, crocus, etc" , "should eventually flower" .)