It's unspeakable (13)
I believe the answer is:
inexpressible
'unspeakable' is the definition.
(similar in meaning)
'it's' is the wordplay.
I cannot quite see how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for inexpressible that I've seen before include "Beyond description" , "Beyond words" .)