Cycling hard? This suggests duty or obligation, perhaps (5)
I believe the answer is:
ought
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for ought that I've seen before include "Have/has a duty (to do)" , "Verb implying duty; zero, nothing (archaic)" , "Be bound" , "Should, had, better" , "outdated anything at all?" .)