Task mostly incorporating one group of singers (5)
I believe the answer is:
choir
'group of singers' is the definition.
(I've seen this before)
'task mostly incorporating one' is the wordplay.
'task' becomes 'chore' ('chore' can be a synonym of 'task').
'mostly' means to remove the last letter.
'incorporating' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'chore' with its last letter removed is 'chor'.
'chor' placed around 'i' is 'CHOIR'.
(Other definitions for choir that I've seen before include "Singing group" , "warblers" , "Band of singers" , "Vocalists in group" , "Carol singers?" .)