The most common and most likely answer to this crossword clue is the 7 letter word SHAKING
I've seen this crossword clue in The Daily Telegraph
S
H
A
K
I
N
G
Buddy explains!

The straight part
We know that "trembling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trembling = shaking.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 SHAKING mean?
SHAKING verb- 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.
- A movement that shakes.