Guarantee an exclamation of surprise (2,4)
I believe the answer is:
my word
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for my word that I've seen before include "Well I never" , "Goodness! - which is my bond!" , "What I would give to guarantee" , "Gosh!" , "This seals promise I make" .)