Taxing (9)
'taxing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for strenuous that I've seen before include "Requiring great exertion" , "urgent" , "Needing forceful exertion" , "Needing much effort" , "Very active" .)
'taxing' is the definition.
(I know that taxing can be written as onerous)
This is all the clue.
(Other definitions for onerous that I've seen before include "Involving a great deal of effort" , "'Heavy, burdensome (7)'" , "Difficult, troublesome" , "Burdensome, demanding" , "weight" .)