It's an offence to be ill, possibly! (5)
I believe the answer is:
libel
'it's an offence' is the definition.
(I've seen this before)
'be ill possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'be'+'ill'='beill'
'beill' is an anagram of 'LIBEL'.
'to' acts as a link.
(Other definitions for libel that I've seen before include "Defamatory publication" , "False and malicious publication to defame" , "Actionable words" , "Written defamation" , "Calumnious writing" .)