Look round a course? Not in this! (3-6)
I believe the answer is:
pea-souper
'not in this' is the definition.
The definition suggests a singular noun which matches the answer.
'look round a course?' is the wordplay.
I cannot quite understand how this works, but
'round' could be 'o' (o is a circular letter) and 'o' is located in the answer.
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the answer.
'course?' could be 's' and 's' is present in the answer.
This explanation may well be incorrect...
(Other definitions for pea-souper that I've seen before include "Dense, dirty fog" , "Thick smokey fog" , "Thick, dirty, yellowish fog" , "Dense yellow fog" .)