The most common and most likely answer to this crossword clue is the 10 letter word SYNONYMOUS
I've seen this crossword clue in The Daily Telegraph
S
Y
N
O
N
Y
M
O
U
S
Buddy explains!
The straight part
We know that "expressing same idea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expressing same idea = synonymous.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 SYNONYMOUS mean?
SYNONYMOUS adjective- (construed with with, narrower sense) having an identical meaning
- (construed with with, broader sense) having a similar meaning
- (construed with with) of, or being a synonym
- (of a SNP) Such that both its forms yield the same sequenced protein.