PC connection lines with marks on both sides (5)
I believe the answer is:
modem
'pc connection' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'lines with marks on both sides' is the wordplay.
'lines' becomes 'ode' (I've seen this before).
'with' says to put letters next to each other.
'marks' becomes 'M' (former German currency).
'on both sides' becomes 'm' (I can't justify this - if you can you should believe this answer much more).
'ode' after 'm' is 'mode'.
'mode'+'m'='MODEM'
Can you help me to learn more?
(Other definitions for modem that I've seen before include "Unit connecting computer to phone line" , "Telephonic computer connection" , "Phone line and computer connector" , "electronic device" , "Data to sound converter" .)