Do pilots need a clearance to go around?
No, and this applies to any pilot, not just an airliner pilot. After the pilot has been cleared to land (or even if you're landing at your own discretion at an untowered airport), he can decide to go around instead of landing if he's dissatisfied with the approach for any reason.

The pilot might need to abort the landing for reasons the tower can't possibly know about, such as a gear unsafe indication, failing to achieve a stabilised approach, not making visual contact with the runway, or some distraction within the cockpit. And even in the case of a reason the tower should be able to tell - such as another aircraft infringing the runway, lining up on the wrong runway by accident, or birds on short final - the pilot must still go around if he thinks it's not safe to land, without waiting to be told.

The same applies to taking off. Even after being cleared to take off, the pilot must not take off if he thinks it is not safe to do so, even if it's for a reason the tower should know about, such as a runway obstruction or a take-off clearance on a crossing runway.