The most common and most likely answer to this crossword clue is the 5 letter word CLING
I've seen this crossword clue in The Daily Telegraph
C
L
I
N
G
More clues leading to the result CLING
Buddy explains!

The straight part
We know that "Hold onto" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hold onto = cling.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 CLING mean?
CLING noun- Fruit (especially peach) whose flesh adheres strongly to the pit.
- Adherence; attachment; devotion
- To hold very tightly, as to not fall off.
- To adhere to an object, without being affixed, in such a way as to follow its contours. Used especially of fabrics and films.
- To cause to adhere to, especially by twining round or embracing.
- To cause to dry up or wither.
- To dry up or wither.
- (with preposition to) to be fond of, to feel strongly about and dependent on
- To produce a high-pitched ringing sound, like a small bell.