Old needing to bend when accepting seat mostly (7)
I believe the answer is:
archaic
'old' is the definition.
(similar in meaning)
'to bend when accepting seat mostly' is the wordplay.
'to bend' becomes 'arc' (arcing is a kind of bending).
'when accepting' is an insertion indicator.
'seat' becomes 'chair' (chair is a kind of seat).
'mostly' means to remove the last letter.
'chair' with its final letter taken away is 'chai'.
'arc' going around 'chai' is 'ARCHAIC'.
'needing' is the link.
(Other definitions for archaic that I've seen before include "Very old or old-fashioned" , "Antiquarian" , "Primitive" , "From the past" , "Belonging to former times like some words" .)