Make mistake with task (6)
I believe the answer is:
errand
'task' is the definition.
(an errand is a task or job)
'make mistake with' is the wordplay.
'make mistake' becomes 'err' (to err is to make an error).
'with' becomes 'and'.
'err'+'and'='ERRAND'
(Other definitions for errand that I've seen before include "run one" , "An ...... of mercy is a journey to relieve distress" , "light task" , "Messenger's job" , "Commission, message" .)