It’s a mark of success (4)
I believe the answer is:
tick
This could be a double definition.
'it's a mark' is the first definition.
(I've seen this before)
'success' is the second definition.
The definition and answer can be both related to natural events as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'of' is the link.
Can you help me to learn more?
(Other definitions for tick that I've seen before include "Instant - credit" , "Little bloodsucker" , "Case of a mattress" , "Check the small mattress?" , "Sound of an old watch" .)