Harbour company probed by Independent columns (7)
I believe the answer is:
portico
'columns' is the definition.
'harbour company probed by independent' is the wordplay.
'harbour' becomes 'port' (I've seen this before).
'company' becomes 'co' (abbreviation).
'probed by' means one lot of letters goes inside another (I've seen this in other clues).
'independent' becomes 'i'.
'port'+'co'='portco'
'portco' placed around 'i' is 'PORTICO'.
(Other definitions for portico that I've seen before include "small architectural feature" , "Entrance with a roof supported by columns" , "series of columns" , "covered area" , "Posh covered entrance to building" .)