Arduous round trip regularly ignored before (6)
I believe the answer is:
tiring
'arduous' is the definition.
(I've seen this in another clue)
'round trip regularly ignored before' is the wordplay.
'round' becomes 'ring' (synonyms).
'regularly ignored' indicates alternate letters (letters removed at regular intervals).
'before' means one lot of letters go next to another.
The alternating letters of 'trip' are 'ti'.
'ring' put after 'ti' is 'TIRING'.
(Other definitions for tiring that I've seen before include "Becoming sleepy" , "Growing weary or bored" , "Causing weariness" , "Boring" , "Exhausting or tedious" .)