The most common and most likely answer to this crossword clue is the 6 letter word ACORNS
I've seen this crossword clue in Independent
A
C
O
R
N
S
Buddy explains!

The straight part
We know that "the start of great things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the start of great things = acorns.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.
What does ACORNS mean?
ACORNS noun- The fruit of the oak, being an oval nut growing in a woody cup or cupule.
- A cone-shaped piece of wood on the point of the spindle above the vane, on the mast-head.
- See acorn-shell.
- (usually in the plural) A testicle.