Mexican Achiote (annatto) Chicken - alternative to achiote This is not a spice that I would normally use.

ACHIOTE



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.
Achiote, commonly known as annatto (https://en.wikipedia.org/wiki/Annatto), is  seed from the achiote fruit. 

The seed is used for coloring more than for flavouring, often to mimic the colour of saffron. The flavour is very subtle - slightly peppery and lemony. 

Saffron would, therefore, be a suitable subsitute for the colour. It would introduce a new flavour to the dish, but generally not one that would be out of place. Alternatively some yellow/orange food coloring or leaving the achiote out altogether would be easy replacements.

Subsitutions for the flavour could be pepper, pink peppercorns or nutmeg, in very small amounts

The spice paste known as 'Achiote paste' is different, but includes achiote seed as a main ingredient. Here (http://www.bonappetit.com/recipe/achiote-paste) is a recipe for it. The annatto in the recipe can be replaced as described above