As long as you don't change the default, it should be able to scan archives (I am talking about pure clam, however clamtk and the rest of frontends use clam in the background, so it should work the same). The setting is configured in /etc/clamd.conf, and the default is to scan archives.
Code:
##
## Archives
##
# ClamAV can scan within archives and compressed files.
# Default: yes
#ScanArchive yes
However, I doubt clam can unpack the archives itself. You might need to install the unrar, unzip, etc. etc. tools.