Include a number with exceptional promise (8)
I believe the answer is:
comprise
'include' is the definition.
(comprising is a kind of including)
'a number with exceptional promise' is the wordplay.
'a number' becomes 'c' (C is an example).
'with' means one lot of letters go next to another.
'exceptional' indicates an anagram.
'promise' anagrammed gives 'omprise'.
'c'+'omprise'='COMPRISE'
(Other definitions for comprise that I've seen before include "Include" , "Be composed of" , "Consist of" .)