Language based on Esperanto is one and the same (3)
I believe the answer is:
ido
'language' is the definition.
(Ido is an example)
'one and the same' is the wordplay.
'one' becomes 'i' (Roman numeral).
'and' says to put letters next to each other.
'the same' becomes 'do' (abbreviation for ditto).
'i'+'do'='IDO'
'based on esperanto is' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ido that I've seen before include "Wedding promise" , "Traditional affirmation to some marriage vows" , "Words made up" , "a language? That's unnatural" , "Wedding response" .)