Mark the highest thing you can see in measuring? (3)
I believe the answer is:
dot
'mark' is the definition.
(dotting is a kind of marking)
'the highest thing you can see in measuring?' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
The remaining letters 'do' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for dot that I've seen before include "Speck or period" , "Long-ago year?" , "Absolutely punctual, on it" , "Morse symbol; spot" , "Small round spot" .)