Red fog seen covering area close to Cairo (6)
I believe the answer is:
maoist
'red' is the definition.
(I've seen this before)
'fog seen covering area close to cairo' is the wordplay.
'fog' becomes 'mist' (synonyms).
'seen covering' is an insertion indicator.
'area' becomes 'a' (maths abbreviation).
'close to' indicates one should take the final letters (the close/ending of).
The final letter of 'cairo' is 'o'.
'a'+'o'='ao'
'mist' going around 'ao' is 'MAOIST'.
(Other definitions for maoist that I've seen before include "supporter of old Communist" , "Political ideologist" , "Chinese-style left-winger" , "Advocate of a Chinese form of communism" , "Chinese communist" .)