What is an ATC clearance? Does a VFR flight have to get a ATC clearance?
All IFR flights require a clearance in controlled (class A-E) airspace. Clearances aren't available in uncontrolled (class F-G) airspace.

VFR flights in certain airspace (ICAO: class B-D, FAA: class B only) require a clearance and may be referred to as CVFR. This clearance will be much simpler than an IFR one, though.

The purpose of an IFR clearance having a limit, route and altitude is so ATC knows what the plane will do if communication is lost and therefore can maintain proper separation of other aircraft. This detail isn't required for a CVFR clearance because a VFR pilot is required to be able to provide his own separation anyway, so the VFR lost comms procedures are much simpler.

ATC can amend a clearance, in which case they only say the items to be changed, or they can cancel the old clearance and issue a new one from scratch. Which they choose depends on how much is changing and why.

An instruction may authorize a (usually brief) deviation from a clearance, e.g. to avoid traffic or weather. ATC can give instructions to VFR flights without a clearance to amend, though, and this doesn't create a new clearance, so they're something different.