The most common and most likely answer to this crossword clue is the 5 letter word GIVER
I've seen this crossword clue in The Guardian
G
I
V
E
R
Buddy explains!

The straight part
We know that "benefactor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case benefactor = giver.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 GIVER mean?
GIVER noun- One who gives; a donor or contributor.