Flying saucers, in the main, are notorious for this. (8)
I believe the answer is:
infamous
'flying' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'saucers in the main are notorious for this' is the wordplay.
I cannot quite see how this works, but
'in' is found in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for infamous that I've seen before include "Notorious, having very bad reputation" , "Of evil repute" , "Disreputable" , "Disgraceful" , "Dishonourable" .)