Talk of a legendary ship lost at last (5)
I believe the answer is:
argot
'talk' is the definition.
(I have seen 'Talk of thieves' mean 'argot' so perhaps 'talk' could also mean 'argot')
'legendary ship lost at last' is the wordplay.
'legendary ship' becomes 'argo' (ship in Greek myth).
'at last' suggests the final letters.
The final letter of 'lost' is 't'.
'argo'+'t'='ARGOT'
'of a' acts as a link.
(Other definitions for argot that I've seen before include "Particular group's jargon" , "manner of speaking" , "Patois, vernacular" , "Jargon or slang (of a group)" , "Groat (anag)" .)