Presumably he likes painstaking people (6)
I believe the answer is:
sadist
'presumably he likes painstaking people' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'presumably he' is the wordplay.
I cannot really see how this works, but
'he' could be 'ad' and 'ad' is found in the answer.
The remaining letters 'sist' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sadist that I've seen before include "Cruel person, takes pleasure in it" , "One taking pleasure in cruelty" , "One who loves being cruel" , "One enjoying inflicting pain" , "One who takes pleasure in cruelty" .)