Howdy,
Presently, org-clock-modify-effort-estimate is not autoloaded. I imagine that org currently suggests/favors workflow would be that you’d clock in, or interact with org-clock some other way, and only then modify your effort estimates.
I found myself wanting to modify a prior effort estimates when not clocked and before the org-clock file had been loaded. This then made me think org-clock-modify-effort-estimate should be autoloaded, and the function act like another entry point.
Best,