Strenuous, sapping (6)
I believe the answer is:
tiring
This clue is a double definition.
'strenuous' is the first definition.
(I've seen this in another clue)
'sapping' is the second definition.
(tiring is a kind of sapping)
(Other definitions for tiring that I've seen before include "Causing weariness" , "Boring" , "Becoming sleepy" , "Wearying, exhausting" , "Growing weary or bored" .)