Not a major underground development (7)
I believe the answer is:
rootlet
'underground development' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rootlet that I've seen before include "Minor growth under a plant" , "Natural lateral support for a climbing plant" , "Small branch of a plant's "anchor"" , "Small radical" , "part vital to growth" .)