Keep barrier in harbour -- copper is checking lines (10)
I believe the answer is:
portcullis
'keep barrier' is the definition.
'portcullis' can be an answer for 'barrier' (portcullis is a kind of barrier). I'm not sure about the 'keep' bit.
'in harbour copper is checking lines' is the wordplay.
'in' means one lot of letters goes inside another.
'harbour' becomes 'port' (I've seen this before).
'copper' becomes 'cu' (chemical symbol).
'checking lines' becomes 'll' (I can't justify this - if you can you should give a lot more credence to this answer).
'port'+'cu'+'is'='portcuis'
'portcuis' enclosing 'll' is 'PORTCULLIS'.
Can you help me to learn more?
(Other definitions for portcullis that I've seen before include "Raisable grating across castle gateway" , "entrance to historic building" , "Sliding grating in castle" , "Grating at castle entrance" , "entry may be barred by this" .)