Earnest op involved artificial tongue (9)
I believe the answer is:
esperanto
'artificial tongue' is the definition.
(I've seen this before)
'earnest op involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'earnest'+'op'='earnestop'
'earnestop' anagrammed gives 'ESPERANTO'.
(Other definitions for esperanto that I've seen before include "Personate (anag.)" , "Parse note in the artificial language" , "Modern language" , "Ape tenors (anag.)" , "special speech" .)