Crossword Genius

Not a major underground development (7)

Ross

I believe the answer is:

rootlet

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'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" .)

I've seen this clue in The Times.
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