Mob following extremely seismic game on board (8)
I believe the answer is:
scrabble
'game on board' is the definition.
(I've seen this before)
'mob following extremely seismic' is the wordplay.
'mob' becomes 'rabble' (synonyms).
'following' says to put letters next to each other.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'seismic' with its middle taken out is 'sc'.
'rabble' after 'sc' is 'SCRABBLE'.
(Other definitions for scrabble that I've seen before include "Board game with tiles" , "In which X=8 and Z=10" , "Scratch - grope - board game" , "Clabbers in popular word game" , "Popular board word-game" .)