My version of Clank! says 2-4 Players on the box and there's no mention of a Solo Mode in the Rulebook. However, on the Renegade Website, it says it's 1-4 Players. Clank! also appears in the 'Solo Mode Included!' sub-section of the 'Shop' section.
So, on their Website it appears (to me, anyway) there's a Solo Mode included in the base game. Does anyone know anything about this? Is this current one a reprint with a Solo Mode added?
I've downloaded the official app for Clank! In! Space! and played it solo. The same app can be used for soloing Clank!
I can only speak for the "in space" version, but it's good fun.
Look for "Renegade Games Companion" in Google Play or the Apple App Store.