Exertion, endeavour (6)
I believe the answer is:
effort
I believe this clue is a double definition.
'exertion' is the first definition.
(I know that exertion can be written as effort)
'endeavour' is the second definition.
(I know that endeavour can be written as effort)
(Other definitions for effort that I've seen before include "Determined attempt" , "Strenuous exertion" , "Endeavour, elbow grease" , "Serious industry - attempt" , "The use of physical or mental energy" .)