Serious consequence of snog at last party (5)
I believe the answer is:
grave
'serious' is the definition.
('grave' can be a synonym of 'serious')
'snog at last party' is the wordplay.
'at last' indicates one should take the final letters.
'party' becomes 'rave' (I've seen this before).
The last letter of 'snog' is 'g'.
'g'+'rave'='GRAVE'
'consequence of' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for grave that I've seen before include "Sleek, black" , "Weighty" , "Solemn in a cemetery?" , "Grim-faced" , "Solemn; tomb" .)