Both promising and charming (8)
I believe the answer is:
engaging
'charming' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for engaging that I've seen before include "embarking on military action" , "Fighting - winning" , "Attractive or charming - getting married?" , "holding fast" , "Winsome" .)