Shirk (associationism) is to direct that which constitutes worship (which takes place with the heart, tongue and limbs) to others besides Allaah.
The action of Shirk is a violation of the belief that there is no creator, provider, sustainer, bringer of benefit and repeller of harm except Allaah alone.
Thus it is a contradiction to direct worship to others besides Allaah in ones actions whilst protesting that one still believes Allaah alone is the creator, provider, sustainer, bringer of benefit and repeller of harm.
This associationism is present amongst the Shia at numerous levels, in #belief and in #worship and it is abundant in their literature.