Well my 2cents worth:
"When approach clearance is obtained from ATC" - approach clearance is the approval for an aircraft to make a specific approach for a specified runway and to descend below MSA or minimum radar vectoring altitude. This approach may be either instrument or visual, but requires the initial clearance from ATC for the pilot to carry out this manoeuvre, due to the reasons that HD stated.
There may be many different types of approach for a single runway, relative to the nav aids available and the procedures as published. Approach clearance is therefore a generic term which covers all of these.
Hope this helps.....
