Lines penned by directors in post (7)
I believe the answer is:
bollard
'post' is the definition.
(bollard is a kind of post)
'lines penned by directors' is the wordplay.
'lines' becomes 'll' (two abbreviations for line).
'penned by' means one lot of letters goes inside another.
'directors' becomes 'board' (board of directors).
'll' placed into 'board' is 'BOLLARD'.
'in' acts as a link.
(Other definitions for bollard that I've seen before include "Mooring post - traffic restriction device" , "Short post barring traffic" , "Post on road or quay" , "Thick post for mooring" , "Strong post" .)