Front bench with 80% leaving — old PM virtually crushed (12)
I believe the answer is:
battleground
'front' is the definition.
(thesaurus)
'leaving old pm virtually crushed' is the wordplay.
'leaving old' becomes 'b' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pm' becomes 'attlee' (prime minister Clement Attlee).
'virtually' means to remove the last letter (virtually all of the word).
'crushed' becomes 'ground' (synonyms).
'attlee' with its last letter taken off is 'attle'.
'b'+'attle'+'ground'='BATTLEGROUND'
'bench with 80%' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for battleground that I've seen before include "Area of conflict" , "scene of conflict" , "Place of military encounter" , "Army hospital near here perhaps" , "arena" .)