The stacking rule for Clash is quite simple: The stronger effect always wins.
For example Rage+Haste would usually result in troops having the damage increase of Rage together with the speed increase of Haste.
Similarly Haste+Iron Fist should result in the speed increase of Haste with the damage increase of Iron Fist.
Rage+Hero ability is an interesting case. Rage has half the damage increase effect on heroes. So Rage+AQ ability probably results in AQ ability damage increase wining out, and rage doing nothing (well, the speed increase would still apply).
"Discrete" spells don't follow this rule, they always stack. This includes Lightning, Heal and Earthquake. They all repeatedly apply a small effect (either heal or damage) at discrete points in time.
I'm not exactly sure how new Poison stacks. It's like a weird mix of a discrete effect and a continuous buff.

