The most common and most likely answer to this crossword clue is the 8 plus 6 letter word RELATIVE CLAUSE
I've seen this crossword clue in The Times
R
E
L
A
T
I
V
E
C
L
A
U
S
E
Buddy explains!
The straight part
We know that "Who could start this?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case who could start this? = relative clause.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.