To abstain, he concludes, is thankless, essentially (6)
I believe the answer is:
desist
'to abstain' is the definition.
('desist' can be a synonym of 'abstain')
'he concludes is thankless essentially' is the wordplay.
'essentially' indicates a hidden word (essence can mean the inside of something).
'DESIST' is hidden in the letters of 'he concludes is thankless'.
(Other definitions for desist that I've seen before include "Stop, cease activity" , "Forbear" , "Stop, abstain" , "Stop, don't continue" , "Abstain, refrain" .)