The most common and most likely answer to this crossword clue is the 3 plus 3 letter word TOM TOM
I've seen this crossword clue in Financial Times
T
O
M
T
O
M
Buddy explains!

The straight part
We know that "Part of rhythm section" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of rhythm section = tom tom.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 TOM TOM mean?
TOM TOM noun- A flat gong (without knob) that is struck with a felt-covered hammer.
- A small joined pair of drums, beaten with the hands.
- (usually as a pair) Any cylindrical drum, with no snare; part of a drum kit.
- To play the tom-tom (pair of drums).