Joke I’d set up and attempt, expressing expert opinion (8)
I believe the answer is:
punditry
'expert opinion' is the definition.
(I've seen this before)
'joke i'd set up and attempt' is the wordplay.
'joke' becomes 'pun' (pun is a type of joke).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'and' means one lot of letters go next to another.
'attempt' becomes 'try' (synonyms).
'id' reversed gives 'di'.
'pun'+'di'+'try'='PUNDITRY'
'expressing' acts as a link.
(Other definitions for punditry that I've seen before include "The ability to speak authoritatively on a subject" , "display of expertise" , "expert opinion" , "The giving of expert opinions" .)