Mark's second (4)
I believe the answer is:
tick
I believe this clue is a double definition.
'mark's' is the first definition.
(thesaurus)
'second' is the second definition.
(I've seen this before)
(Other definitions for tick that I've seen before include "Mark of work done (right)" , "Sound of an old watch" , "Credit (sl.)" , "Case of a mattress" , "Mark indicating correctness" .)