Select a number with no factors (5)
I believe the answer is:
prime
'select' is the definition.
(synonyms)
'a number with no factors' is the wordplay.
I cannot really understand how this works, but
'number' could be 'm' (M is an example) and 'm' is found within the answer.
The remaining letters 'prie' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for prime that I've seen before include "Main - charge" , "Foremost" , "put in the picture" , "Two or three, for example" , "Best quality, perhaps of beef" .)