Games area becomes essentially shrouded in great, thick fog (3-6)
I believe the answer is:
pea souper
'thick fog' is the definition.
(I've seen this before)
'games area becomes essentially shrouded in great' is the wordplay.
'games area' becomes 'pea' (I can't justify this - if you can you should give a lot more credence to this answer).
'essentially' says to take the centre.
'shrouded in' is an insertion indicator.
'great' becomes 'super' (both can mean excellent).
The central letter of 'becomes' is 'o'.
'o' going within 'super' is 'souper'.
'pea'+'souper'='PEA-SOUPER'
Can you help me to learn more?
(Other definitions for pea souper that I've seen before include "A really dense fog of leguminous kind" , "Thick, dirty, yellowish fog" , "Dense, dirty yellow fog" , "Thick smokey fog" , "Thick yellow fog of old" .)