Old seat almost brought into curving line (7)
I believe the answer is:
archaic
'old' is the definition.
(similar in meaning)
'seat almost brought into curving line' is the wordplay.
'seat' becomes 'chair' (chair is a kind of seat).
'almost' means to remove the last letter (almost the whole word is kept).
'brought into' means one lot of letters goes inside another.
'curving line' becomes 'arc' (I've seen this in another clue).
'chair' with its last letter removed is 'chai'.
'chai' put inside 'arc' is 'ARCHAIC'.
(Other definitions for archaic that I've seen before include "No longer in general use" , "A long way out of date" , "Antiquarian" , "Of a much earlier period" , "Ancient or out of date" .)