Main office? Grace tucked in chairs (12)
I believe the answer is:
headquarters
'main office?' is the definition.
(I know that main office can be written as headquarters)
'grace tucked in chairs' is the wordplay.
'grace' becomes 'quarter' (quarter can mean grace or tolerance).
'tucked in' means one lot of letters goes inside another (I've seen this in other clues).
'chairs' becomes 'heads' (I've seen this in another clue).
'quarter' put into 'heads' is 'HEADQUARTERS'.
(Other definitions for headquarters that I've seen before include "Military command point" , "Operational centre" , "Administrative centre" , "Base" , "Military command post" .)