Not allowed to be on the sick list when I caught it (7)
I believe the answer is:
illicit
'not allowed' is the definition.
(illicit means against the law)
'on the sick list when i caught it' is the wordplay.
'on the sick list' becomes 'i'll' (I've seen this in another clue).
'when i' becomes 'ci' (I can't justify this - if you can you should give a lot more credence to this answer).
'caught' means one lot of letters goes inside another.
'ci' going inside 'it' is 'icit'.
'ill'+'icit'='ILLICIT'
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for illicit that I've seen before include "wrong" , "Contraband" , "Not lawful" , "illegitimate" , "Contrary to morality or convention" .)