Present silly, but not sick, unorthodox opinion (6)
I believe the answer is:
heresy
'unorthodox opinion' is the definition.
(I've seen this before)
'present silly but not sick' is the wordplay.
'present' becomes 'here' (both can mean in attendance).
'but not' is a deletion indicator.
'sick' becomes 'ill' (synonyms).
'silly' with 'ill' taken out is 'sy'.
'here'+'sy'='HERESY'
(Other definitions for heresy that I've seen before include "Iconoclasm" , "uncommon view" , "Divergence from orthodoxy" , "Unorthodox religious belief" , "Opinion opposed to conventional belief" .)