Avoid including 500 fellows in one group promotion (13)
I believe the answer is:
advertisement
'promotion' is the definition.
(advertisement is a kind of promotion)
'avoid including 500 fellows in one group' is the wordplay.
I cannot really see how this works, but
'500' could be 'd' (Roman numeral for 500) and 'd' is present in the answer.
'avoid' could be 'avert' (synonyms) and 'avert' is present in the remaining letters.
'fellows' could be 'men' (fellow can mean a man) and 'men' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'group' could be 'set' (both can mean a collection) and 'set' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for advertisement that I've seen before include "trailer" , "plug" , "Public promotion" , "Notice - commercial" , "Publicity announcement" .)