Harbour an officer in the porch (7)
I believe the answer is:
portico
'the porch' is the definition.
(portico is a kind of porch)
'harbour an officer' is the wordplay.
'harbour' becomes 'port' (I've seen this before).
'an' becomes 'i' (Roman numeral for one).
'officer' becomes 'co' (Commanding Officer abbreviation).
'port'+'i'+'co'='PORTICO'
'in' acts as a link.
(Other definitions for portico that I've seen before include "gallery, perhaps" , "Grand entrance" , "Covered entrance with columns" , "small architectural feature" , "Covered walkway" .)