Enchant creature
Enchanted creature gets +1/+1 and has "Whenever this creature attacks, tap target creature defending player controls."
The enchanted creature is the source of the triggered ability it gains, not Grasp of the Hieromancer. If the enchanted creature isn’t white, that ability can target a creature with protection from white, for example.
The triggered ability granted to the enchanted creature will tap the creature before blockers are declared.