As quickly as letters are delivered? (9)
I believe the answer is:
posthaste
'as quickly' is the definition.
I can't tell whether this defines the answer.
'letters are delivered?' is the wordplay.
'letters' becomes 'post' (letters in the mail or post).
'are delivered?' becomes 'haste' (I can't justify this - if you can you should believe this answer much more).
'post'+'haste'='POSTHASTE'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for posthaste that I've seen before include "Yes!" , "at the double" , "as fast as possible" , "With particular speed" , "Like Bat out of Hell" .)