Sponsorship: something valuable with added interest (7)
I believe the answer is:
auspice
'sponsorship' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for auspice that I've seen before include "future promise" , "Omen; epic USA (anag.)" , "Portent" , "Omen - I use cap (anag)" , "Omen (archaic)" .)