... Saunders essentially going on second after support is rejected - it's famously hard (7)
I believe the answer is:
diamond
'it's famously hard' is the definition.
(diamond is a very hard material)
'saunders essentially going on second after support is rejected' is the wordplay.
'essentially' indicates the central letters.
'going on' says to put letters next to each other.
'second after' becomes 'om' (I am not sure about this - if you are sure you should believe this answer much more).
'support' becomes 'aid' (to aid is to help or support).
'is rejected' shows that the letters should be reversed in order.
The centre of 'saunders' is 'nd'.
'om'+'aid'='omaid'
'omaid' written backwards gives 'diamo'.
'nd' put after 'diamo' is 'DIAMOND'.
(Other definitions for diamond that I've seen before include "Very hard substance" , "Gemstone - baseball field" , "Form of carbon" , "60th anniversary" , "Small type" .)