Old-fashioned seat mostly fitted in new car (7)
I believe the answer is:
archaic
'old-fashioned' is the definition.
(I've seen this before)
'seat mostly fitted in new car' is the wordplay.
'seat mostly' becomes 'chai' (I can't explain this - if you can you should believe this answer much more).
'fitted in' is an insertion indicator.
'new' is an anagram indicator.
'car' is an anagram of 'arc'.
'chai' going within 'arc' is 'ARCHAIC'.
Can you help me to learn more?
(Other definitions for archaic that I've seen before include "Extremely old" , "From the past" , "Ancient, no longer current" , "Antiquarian" , "Belonging to former times like some words" .)