Rash's gone away on bust (8)
I believe the answer is:
outbreak
'rash's' is the definition.
(outbreak can mean a rash or flare-up)
'away on bust' is the wordplay.
'away' becomes 'out' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'bust' becomes 'break' (synonyms).
'out'+'break'='OUTBREAK'
'gone' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for outbreak that I've seen before include "Violent occurrence" , "Sudden occurrence (of war, disease, etc.)" , "Sudden development of disease etc" , "rash" , "disturbance" .)