Least modest (7)
I believe the answer is:
vainest
'least modest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vainest that I've seen before include "Most proud" , "Arrogant in the extreme" , "Most fond of one's own looks" , "emptier than all the others" , "Most conceited" .)