Ancient language found in the middle of clay altar plinth (5)
I believe the answer is:
latin
'ancient language' is the definition.
(Latin is an ancient language)
'found in the middle of clay altar plinth' is the wordplay.
'found in' indicates an anagram.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'middle of' says to take the centre.
'altar plinth' becomes 'in' (I can't justify this - if you can you should give a lot more credence to this answer).
The central letters of 'clay' are 'la'.
't'+'la'='tla'
'tla' with letters rearranged gives 'lat'.
'lat'+'in'='LATIN'
Can you help me to learn more?
(Other definitions for latin that I've seen before include "A Romance language" , "Language of old 8 down" , "Shakespeare's was small, according to Jonson" , "and 15 Down: Student area of Paris" , "The language of old Rome" .)