Worthless setter's endlessly in trouble (7)
I believe the answer is:
trivial
'worthless' is the definition.
(thesaurus)
'setter's endlessly in trouble' is the wordplay.
'setter's endlessly' becomes 'vi' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'trouble' becomes 'trial' (trial is a kind of trouble).
'vi' going into 'trial' is 'TRIVIAL'.
Can you help me to learn more?
(Other definitions for trivial that I've seen before include "Piddling" , "Worthless" , "Popular board game" , "Of little substance or significance" , "Of little consequence" .)