If the field "clientip_city" contains the IP addresses, you can try this search instead:
SRC=* clientip_city=* | geoip clientip_city
Update:
To extract the field, you have to either configure the fields to be extracted (see http://www.splunk.com/base/Documentation/latest/Knowledge/Addfieldsatsearchtime) or extract them inline:
SRC=* | rex "(?<ip>\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})" | geoip ip
This will extract the first matching IP in the event.
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