Pass on, leaving item of punctuation (3)
I believe the answer is:
col
'pass' is the definition.
(col is a term for a mountain pass)
'on leaving item of punctuation' is the wordplay.
'leaving' is a deletion indicator.
'item of punctuation' becomes 'colon'.
'colon' with 'on' taken away is 'COL'.
(Other definitions for col that I've seen before include "Low point of a ridge" , "Pass between mountain ridges" , "A pass between mountain peaks" , "Depression in range of mountains" , "Pass between hills" .)