Senseless idea not alerting new entrants first of all (5)
I believe the answer is:
inane
'senseless' is the definition.
(thesaurus)
'idea not alerting new entrants first of all' is the wordplay.
'first of all' indicates taking the first letters.
The first letters of 'idea not alerting new entrants' is 'INANE'.
(Other definitions for inane that I've seen before include "unimaginitive" , "Annie (anag.)" , "Idiotic" , "Fatuous, mindless" , "Vacuous, silly" .)