Symptom of a cold caught an hour after noon - go away! (7)
I believe the answer is:
atishoo
'symptom of a cold caught an hour' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'noon go away' is the wordplay.
'noon' becomes 'ati' (I can't justify this - if you can you should believe this answer much more).
'go away' becomes 'shoo' (I've seen this before).
'ati'+'shoo'='ATISHOO'
'after' is the link.
Can you help me to learn more?
(Other definitions for atishoo that I've seen before include "Sudden eruption" , "Word to be sneezed at" , "hay-fever sufferer may interject this" , "Sound of explosion" , "Sneezing sound" .)