This is not a spice that I would normally use.
However I was given some and then looked for how to use it. Food52 has this to say about it:
Achiote paste is a beautiful blend of spices which give the chicken a wonderful complex flavor.
(Great recipe here) I became a bit suspicious of the spice's freshness, so threw it out - however would like to make the Mexican chicken - but need an alternative to achiote. The question being: What spice can I use instead of achiote to achieve a similar flavor?
It may be more commonly known as 'Annatto' - see answer below.