About to sit down, initially rested (7)
I believe the answer is:
reposed
'rested' is the definition.
('repose' can be a synonym of 'rest')
'about to sit down initially' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to sit' becomes 'pose' (as in sitting for a painter).
'initially' says to take the initial letters.
The first letter of 'down' is 'd'.
're'+'pose'+'d'='REPOSED'
(Other definitions for reposed that I've seen before include "What's settled" , "Relaxed" , "Rested" , "modelled afresh" .)