A predicament shows promise (6)
I believe the answer is:
plight
This clue is a double definition.
'a predicament' is the first definition.
(I know that predicament can be written as plight)
'promise' is the second definition.
(I know that plight is a more specific form of the action the vow)
'shows' acts as a link.
(Other definitions for plight that I've seen before include "Pledge; predicament" , "Serious predicament" , "A serious or difficult situation" , "Promise - trouble" , "Dangerous or difficult situation" .)