In essence, hard boring task (5)
I believe the answer is:
chore
'boring task' is the definition.
(I've seen this before)
'in essence hard' is the wordplay.
'in' is an insertion indicator.
'essence' becomes 'core' ('core' can be a synonym of 'essence').
'hard' becomes 'h'.
'core' enclosing 'h' is 'CHORE'.
(Other definitions for chore that I've seen before include "Drudgery" , "easy option? Not so!" , "Boring task" , "Stream's murmuring" , "burden" .)