Take ill before appearing in and you stand in for (9)
I believe the answer is:
represent
'stand in for' is the definition.
(thesaurus)
'take ill before appearing in' is the wordplay.
'take ill' becomes 'resent' (I can't explain this - if you can you should believe this answer much more).
'before' becomes 'pre' (prefix meaning 'before').
'appearing in' indicates putting letters inside.
'resent' enclosing 'pre' is 'REPRESENT'.
'and you' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for represent that I've seen before include "Serve as a symbol for" , "declare" , "Mean" , "Portray" , "Explain" .)