Excellent number with no factors (5)
I believe the answer is:
prime
'excellent' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for prime that I've seen before include "Prepare to set off" , "Maybe eleven" , "Foremost" , "Best - get ready" , "Best quality, perhaps of beef" .)