Rely on this unwritten promise? Goodness! (4,2,4)
I believe the answer is:
upon my word
'rely on this' is the definition.
(both can be said to show something is true)
'unwritten promise? goodness' is the wordplay.
I cannot really understand how this works, but
'promise?' could be 'word' (as in 'I give you my word') and 'word' is found in the answer.
'goodness' could be 'my' (both are exclamations of surprise) and 'my' is found within the answer.
The remaining letters 'upon' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for upon my word that I've seen before include "You surprise me" , "Heavens" , "when I tell you to!" .)