An exhausting thing to do, though the purpose is lofty (3,2)
I believe the answer is:
use up
'an exhausting thing to do though' is the definition.
I can't judge whether this definition defines the answer.
'purpose is lofty' is the wordplay.
I cannot really see how this works, but
'purpose' could be 'use' (synonyms) and 'use' is found within the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'the' is the link.
Can you help me to learn more?
(Other definitions for use up that I've seen before include "Exhaust (resources)" , "Entirely consume" , "Consume (stocks)" , "Finish, exhaust" , "Expend" .)