Bottom on a chair in the end (4)
I believe the answer is:
rear
'bottom' is the definition.
('rear' can be a synonym of 'bottom')
'on a chair in the end' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to'**).
'in the end' says to take the final letters.
The final letter of 'chair' is 'r'.
're'+'a'+'r'='REAR'
(Other definitions for rear that I've seen before include "The back part of something" , "Of horse, rise up on hind legs" , "Buck" , "Rump" , "Bring up or stand up on hind legs" .)