Briefly ill, arrive not wanted (9)
I believe the answer is:
unwelcome
'briefly ill arrive' is the definition.
I can't tell whether this defines the answer.
'not wanted' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'wanted' becomes 'welcome' ('welcome' can be similar in meaning to 'wanted'**).
'un'+'welcome'='UNWELCOME'
Can you help me to learn more?
(Other definitions for unwelcome that I've seen before include "not willingly admitted" , "No received with pleasure - by host?" , "Not gladly received" , "Bad (news); not acceptable" , "displeasing!" .)