Language, bound to be speechless (6-4)
I believe the answer is:
tongue-tied
'be speechless' is the definition.
(thesaurus)
'language bound' is the wordplay.
'language' becomes 'tongue' (tongue can mean a language).
'bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'tongue'+'tied'='TONGUE-TIED'
'to' is the link.
(Other definitions for tongue-tied that I've seen before include "Unable to say" , "Speechless from embarrassment" , "Speechless with shyness" , "(Out of shyness) barely able to speak" , "Unable to speak coherently" .)