During an instrument approach, what are the obstruction clearance given?
The FAA lays this out in Order 8260.3c the three main relevant sections for requirements, level flight and climbing/decent segments are:


  2-1-2. Required Obstacle Clearance (ROC). This order specifies the minimum measure of obstacle clearance considered by the FAA to supply
  a satisfactory level of vertical protection. The validity of the
  protection is dependent, in part, on assumed aircraft performance. In
  the case of TERPS, it is assumed that aircraft will perform within
  certification requirements.


...


  2-1-3. Level OCS. The level OCS concept is applicable to “level
  flight” segments. These segments are level flight operations intended
  for en route, STAR, feeder, initial, intermediate, and NPA final
  approach segments. A single ROC value is applied over the length of
  the segment. These values were determined through testing and
  observation of aircraft and pilot performance in various flight
  conditions. Typical ROC values are: 1000 feet (2000 over designated
  mountainous terrain) for en route, STAR, and feeder segments, 1000
  feet for initial segments, 500 feet for intermediate segments, and
  250-500 feet for final segments.


...


  2-1-4. Sloping OCS. The method of applying ROC, in segments dedicated
  to descending on a glidepath or climbing in a departure or missed
  approach segment, requires a different obstacle clearance concept than
  the level OCS because the ROC value must vary throughout the segment.
  The value of ROC near the runway is relatively small and increases
  throughout the segment.


The document is lengthy and lays out various formulas and charts, to keep form copying the whole thing here please reference the document directly for anything more specific.