Mark left working behind company (5)
I believe the answer is:
colon
'mark' is the definition.
(I know that colon is a type of punctuation)
'left working behind company' is the wordplay.
'left' becomes 'l' (common abbreviation).
'working' becomes 'on' (on can mean working or activated).
'behind' means one lot of letters go next to another.
'company' becomes 'co' (abbreviation).
'l'+'on'='lon'
'lon' after 'co' is 'COLON'.
(Other definitions for colon that I've seen before include ": - bit of the gut" , "Stop (less than full)" , "Panama port; part of intestine" , "Part of intestines" , "cut short dependency?" .)