The most common and most likely answer to this crossword clue is the 8 plus 5 letter word SOUNDING BOARD
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "off whom one can bounce ideas?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case off whom one can bounce ideas? = sounding board.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 SOUNDING BOARD mean?
SOUNDING BOARD noun- A thin board that forms part of the resonating chamber of a musical instrument and serves to reinforce its sound
- A structure that reflects sound towards the intended listeners
- (by extension) Any device or means used to spread an idea or point of view
- A person, or group, whose reactions to a new idea or proposal serve to assess its acceptability