A king acquired the language of a particular class (5)
I believe the answer is:
argot
'language of a particular class' is the definition.
(argot means the language used by a certain group)
'a king acquired' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'acquired' becomes 'got' ('get' can be a synonym of 'acquire').
'a'+'r'+'got'='ARGOT'
'the' is the link.
(Other definitions for argot that I've seen before include "Slang of a particular group, originally thieves" , "Talk of thieves" , "Groat (anag)" , "Particular group's jargon" , "Father or mother" .)