The most common and most likely answer to this crossword clue is the 10 letter word EXPRESSIVE
I've seen this crossword clue in The Daily Telegraph
E
X
P
R
E
S
S
I
V
E
Buddy explains!
The straight part
We know that "Significant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case significant = expressive.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 EXPRESSIVE mean?
EXPRESSIVE noun- Any word or phrase that expresses (that the speaker, writer, or signer has) a certain attitude toward or information about the referent.
- (more narrowly) A word or phrase, belonging to a distinct word class or having distinct morphosyntactic properties, with semantic symbolism (for example, an onomatopoeia), variously considered either a synonym, a hypernym or a hyponym of ideophone.
- Effectively conveying thought or feeling.