Is it legal for a A/C to not have some kind of clock to measure MR time.
Yes.
Ie is it possible for the pilot to "guesstimate" this time?
Yes, of course it’s “possible”, but if the pilot’s “guesstimate” is wrong, there will be a breach of the obligation to record actual time in service.
At the end of a day’s flying a pilot can decide how much TIS to record in the MR by throwing a dart at a dart board. Let’s say she hits ‘19’ and decides to add 1.9 hours’ TIS to the MR.
It could be that by pure coincidence the aircraft spent 1.9 hours in the air (not on jacks) that day, in which case the MR reflects the aircraft’s TIS. If, on the other hand, the aircraft spent 2.9 hours in the air that day, adding 1.9 hours TIS on the MR will not reflect the actual TIS.
TIS is an objective fact. If the MR does not reflect the aircraft’s actual TIS at the end of each day’s flying, there is a breach.
That’s why the ‘dart board’ or ‘guesstimate’ methods are not recommended, but the methods referred to by Old Akro and wishiwasupthere are.
If a bunch of different pilots fly an aircraft without an air-switch and no one’s sure how much time in total the aircraft spent in the air, you have a problem. The only practical solution is to find out what each pilot entered in his or her logbook, noting that loggable flight time for flight crew is usually longer than TIS, and estimate TIS from there.
Entering an estimate is better than entering nothing.
Over-estimating is better than under-estimating. The regulator gets a little attentive when an aircraft with only 5 hours’ TIS for a day was flown by pilots who logged 10.