Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.Agent.CLK is a detection for a certain type of Trojan that belongs to the Adware.Adclick family. This family uses scheduled tasks to show user advertisements. It also collects information from the browser(s) and connects to an IRC server.
Offered at crack and keygen sites under various names to trick users into downloading and running it.
Malwarebytes can remove Trojan.Agent.CLK without further user interaction.