Assurance given by yours truly, well! (2,4)
I believe the answer is:
my word
'assurance given by yours truly well' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'assurance given by yours truly well' is the wordplay.
I cannot really understand how this works, but
'assurance' could be 'word' (I've seen this in another clue) and 'word' is present in the answer.
'well' could be 'my' (I've seen this in other clues) and 'my' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for my word that I've seen before include "This seals promise I make" , "Good heavens" , "The gift is a promise" , "Well I'm blowed!" , "Former radio programme; I am surprised!" .)