It stands still and marks time (6)
I believe the answer is:
gnomon
'it stands still' is the definition.
The definition suggests a singular noun which matches the answer.
'marks time' is the wordplay.
I cannot really see how this works, but
'marks' could be 'm' (former German currency) and 'm' is found within the answer.
'time' could be 'noon' (noon is a kind of time) and 'noon' is present in the leftover letters.
A single letter 'g' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'and' acts as a link.
Can you help me to learn more?
(Other definitions for gnomon that I've seen before include "where time casts its shadow" , "Pin of a sundial whose shadow points to the hour" , "Sundial rod" , "Sundial arm" , "Part of a sundial projecting a shadow" .)