Not seriously concerned with viewer getting covered in jelly, oddly (9)

'not seriously' is the definition.
(jocular means joking or not serious)

'concerned with viewer getting covered in jelly oddly' is the wordplay.
'concerned with viewer' becomes 'ocular' (relating to eyes - organs that view things).
'getting covered in' indicates putting letters inside.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'jelly' are 'jly'.
'ocular' going into 'jly' is 'JOCULARLY'.