One involved in endless task that is boring vocal group (5)
I believe the answer is:
choir
'vocal group' is the definition.
(group of vocalists)
'one involved in endless task' is the wordplay.
'one' becomes 'i' (Roman numeral).
'involved in' indicates putting letters inside.
'endless' means to remove the last letter.
'task' becomes 'chore' (synonyms).
'chore' with its last letter removed is 'chor'.
'i' placed into 'chor' is 'CHOIR'.
'that is boring' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for choir that I've seen before include "Large group of singers" , "set of singers" , "Vocal group" , "Singing group" , "They sing" .)