Good evening, alasdair13.
28A - anagram of O(ring) IT, and C (chic at the bottom) Answer means "of the ear"
51a. anagram of CHI (Greek letter X) and LESS - answer could also mean "tools".
56 - abbreviation for Force, plus 3 directions from the compass.
81. You are right - eight is a cube, inside hen (a layer of eggs)
100A. Sorry, but your "S" is wrong. Answer is anagram of R(iver), ESK, and SAND - meaning "shade" as in absence of light.
Hoping this helps1
Al Issner