The most common and most likely answer to this crossword clue is the 5 letter word SHAKE
I've seen this crossword clue in The Guardian
S
H
A
K
E
More clues leading to the result SHAKE
- Fish found beneath small rock
- Radical change sees tribal leader quoted in court
- Shuffles fish during drinks
- A second milky drink
- Henry tucks into foreign tea, a milky drink
- Small fish, rock
- Small fish is upset
- Twitch from fish on top of slab
- Have evening meal traditionally including fish for a change
- Unnerve Arab ruler in audience
Buddy explains!

The straight part
We know that "rock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rock = shake.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 SHAKE mean?
SHAKE noun- The act of shaking or being shaken; tremulous or back-and-forth motion.
- A milkshake.
- A beverage made by adding ice cream to a (usually carbonated) drink; a float.
- Shake cannabis, small, leafy fragments of cannabis that gather at the bottom of a bag of marijuana.
- (building material) A thin shingle.
- A crack or split between the growth rings in wood.
- A fissure in rock or earth.
- A basic wooden shingle made from split logs, traditionally used for roofing etc.
- Instant, second. (Especially in two shakes.)
- One of the staves of a hogshead or barrel taken apart.
- A rapid alternation of a principal tone with another represented on the next degree of the staff above or below it; a trill.
- A shook of staves and headings.
- The redshank, so called from the nodding of its head while on the ground.
- To cause (something) to move rapidly in opposite directions alternatingly.
- To move (one's head) from side to side, especially to indicate refusal, reluctance or disapproval.
- To move or remove by agitating; to throw off by a jolting or vibrating motion.
- To disturb emotionally; to shock.
- To lose, evade, or get rid of (something).
- To move from side to side.
- (usually as "shake on") To shake hands.
- To dance.
- To give a tremulous tone to; to trill.
- To threaten to overthrow.
- To be agitated; to lose firmness.