Lost art, say, that's been restored (6)
I believe the answer is:
astray
'lost' is the definition.
(synonyms)
'art say that's been restored' is the wordplay.
'that's been restored' indicates anagramming the letters.
'art'+'say'='artsay'
'artsay' with letters rearranged gives 'ASTRAY'.
(Other definitions for astray that I've seen before include "Going wrong" , "Off the correct track" , "A satyr (anag.)" , "Gone off the right path" , "in the wrong place" .)