The most common and most likely answer to this crossword clue is the 5 plus 4 letter word TROUT FARM
I've seen this crossword clue in The Daily Telegraph
T
R
O
U
T
F
A
R
M
Buddy explains!
The straight part
We know that "Where to see the beginning — and end — of the rainbow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where to see the beginning — and end — of the rainbow = trout farm.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.