The function open-tcp-stream connects to a server and returns a stream (of type socket-stream). The stream is then used for communication through the socket using the standard Lisp I/O functions.
connect-to-tcp-server can also be used, especially if you want to subclass socket-stream.
For examples illustrating simple write and read on a socket, see these files in lib/7-1-0-0/examples
capi/applications/chat.lisp
capi/applications/chat-client.lisp
LispWorks User Guide and Reference Manual - 20 Sep 2017