Exertion (6)
I believe the answer is:
effort
'exertion' is the definition.
(I know that exertion can be written as effort)
This is the entire clue.
(Other definitions for effort that I've seen before include "'Endeavour, exertion (6)'" , "Serious industry - attempt" , "For eft (anag.)" , "Attempt - hard work" , "Struggle" .)