The most common and most likely answer to this crossword clue is the 9 letter word ESPERANTO
I've seen this crossword clue in Independent
E
S
P
E
R
A
N
T
O
More clues leading to the result ESPERANTO
- Guide operates with northern language
- Pet reason for developing an artificial language
- Language operates poorly without noun
- Extraordinary sense finally awoken in one inspiring language
- Neat prose could be translated into this
- Tongue not working after period of telepathy?
- Stifling communication in the end, doctor operates — words rarely spoken
- Tongue in pain ultimately, doctor operates to sew it up
- Ten operas translated into language that’s artificial
- Fourth letter in Oscar Peterson’s made-up language
Buddy explains!
The straight part
We know that "Synthetic tongue" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case synthetic tongue = esperanto.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.