Prepared the supports for the seats (7)
I believe the answer is:
settees
'the seats' is the definition.
(settee is a kind of seat)
'prepared the supports' is the wordplay.
'prepared' becomes 'set' (set up or ready).
'the supports' becomes 'tees' (tee is a kind of support).
'set'+'tees'='SETTEES'
'for' acts as a link.
(Other definitions for settees that I've seen before include "furniture" , "Long indoor seats" , "Sofas" .)