Demanding, challenging (8)
I believe the answer is:
exacting
I believe this is a double definition.
'demanding' is the first definition.
(exacting is a kind of demanding)
'challenging' is the second definition.
(I've seen this before)
(Other definitions for exacting that I've seen before include "Making great demands like difficult task" , "Unreasonable in making demands" , "Making rigorous demands" , "Requiring hard work and attention" , "Harsh" .)