Astonished that it's lacking an address (10)
I believe the answer is:
speechless
'astonished that it's lacking an address' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs occasionally mean the same thing.
'astonished that it's lacking' is the wordplay.
'astonished' becomes 'speech' (I can't explain this - if you can you should give a lot more credence to this answer).
'that it's lacking' becomes 'less' (I've seen this in another clue).
'speech'+'less'='SPEECHLESS'
Can you help me to learn more?
(Other definitions for speechless that I've seen before include "Struck dumb" , "Unable to say anything" , "Dumbfounded" , "In shock" , "being forced to improvise?" .)