Rising benefit upset expelled head of Thatcherite element (7)
I believe the answer is:
sulphur
'element' is the definition.
(I know that sulphur is a type of element)
'rising benefit upset expelled head' is the wordplay.
'rising' says the letters should be written in reverse (going up in a down clue).
'benefit' becomes 'plus' (both can mean an advantageous feature).
'upset' is a reversal indicator.
'expelled head' becomes 'ruh' (I can't explain this - if you can you should believe this answer much more).
'plus' in reverse letter order is 'sulp'.
'ruh' backwards is 'hur'.
'sulp'+'hur'='SULPHUR'
'of thatcherite' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sulphur that I've seen before include "Element, atomic no. 16" , "Chemical element" , "Brimstone" , "Yellow non-metallic element" , "S (chem.)" .)