Column with a line related to mail (6)
I believe the answer is:
postal
'related to mail' is the definition.
(I know that postal means to do with or relating to mail)
'column with a line' is the wordplay.
'column' becomes 'post' (both can mean an upright support).
'with' is a charade indicator (letters next to each other).
'line' becomes 'l' (used when specifying particular lines from a poem).
'post'+'a'+'l'='POSTAL'
(Other definitions for postal that I've seen before include "to go so mad in America" , "To do with snail mail" , "Type of order" , "Relating to mail delivery" , "To Alps (anag.)" .)