Ancient ring left on lair (5)
I believe the answer is:
olden
'ancient' is the definition.
(I've seen this in another clue)
'ring left on lair' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'left' becomes 'l' (common abbreviation).
'on' means one lot of letters go next to another.
'lair' becomes 'den' (synonyms).
'o'+'l'+'den'='OLDEN'
(Other definitions for olden that I've seen before include "Of a former age" , "Led on (anag)" , "as in time gone by" , "of ancient history?" , "Bygone" .)