Upset about former PM's railway support (7)
I believe the answer is:
sleeper
'support' is the definition.
(I know that sleeper is a type of brace)
'upset about former pm's railway' is the wordplay.
'upset' says the letters should be written backwards.
'about former' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'pm's' becomes 'Peel's' (e9th-century prime minister Robert Peel).
'railway' becomes 'r' (eg in Great Western Railway).
'e'+'peels'='epeels'
'epeels' backwards is 'sleepe'.
'sleepe'+'r'='SLEEPER'
Can you help me to learn more?
(Other definitions for sleeper that I've seen before include "Railway line support" , "One who's out" , "Timber beam" , "Kipper" , "Beam across railway track or berth on ship" .)