Annoyance of stupid endless train trip (8)
I believe the answer is:
irritant
'annoyance' is the definition.
(I know that irritant is a type of annoyance)
'stupid endless train trip' is the wordplay.
'stupid' indicates an anagram (in a stupid order).
'endless' means to remove the last letter.
'train'+'trip'='traintrip'
'traintrip' with its last letter removed is 'traintri'.
'traintri' with letters rearranged gives 'IRRITANT'.
'of' acts as a link.
(Other definitions for irritant that I've seen before include "distraction" , "Annoying factor" , "causing annoyance" , "Annoying thing" , "Something that causes annoyance and bad temper" .)