Left a top thesis in a very old language (5)
I believe the answer is:
latin
'old language' is the definition.
(Latin is a dead language)
'left a top thesis in' is the wordplay.
'left' becomes 'l' (common abbreviation).
'top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letter of 'thesis' is 't'.
'l'+'a'+'t'+'in'='LATIN'
'a very' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for latin that I've seen before include "foreign language" , "'Dead language, though still taught (5)'" , "Language of ancient Rome" , "Hispanic, for example" , "Virgil's tongue" .)