Post card containing two lines (7)
I believe the answer is:
bollard
'post' is the definition.
(I know that bollard is a type of post)
'card containing two lines' is the wordplay.
'card' becomes 'board' (synonyms).
'containing' means one lot of letters goes inside another.
'two lines' becomes 'll' (two abbreviations for line).
'board' going around 'll' is 'BOLLARD'.
(Other definitions for bollard that I've seen before include "Thick post for mooring" , "Roadside post" , "Short post barring traffic" , "Strong post" , "Mooring post - traffic restriction device" .)