The most common and most likely answer to this crossword clue is the 7 letter word ROOTLET
R
O
O
T
L
E
T
More clues leading to the result ROOTLET
- Search vigorously over time for minor radical
- Marsupial allowed to eat top of this plant part
- Grub served on Tube, at the outset small underground branch
- Reportedly path allowed, part going underground
- Both hands grapple excessively redirecting alien plant section
- Search vigorously given time to find minor radical
- Mentioned way to allow a bit of underground development
- Said method allowed to get part of a plant
Buddy explains!

The straight part
We know that "bit of underground development" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bit of underground development = rootlet.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.