a person who finds that the only way to uphold the ideals and principles of one's belief system or group memberships (churches, clubs, political parties, socioeconomic groups, etc.) is to BREAK its rules mindfully. THis means the person does so knowing full well what the repercussions and backlash for his/her actions or words might be. While the word heresy is rooted in lots of religious history, a mindful heretic is possible not just in religious organizations, but also in broader secular contexts as well.
A family has a tradition that has existed for generations tjat depends on a racist pattern of behavior. When you choose to break with that tradition because you can not in good faith continue being a member of the family unless you refuse to follow the tradition, even if you know you won't be invited to family gatherings, you have been a mindful heretic to your family's orthodoxy.
A theologian who chooses to write what s/he believes to be true despite the religious consequences is also a mindful heretic. See Matthew Fox's Confessions of a Post-Denominational Priest for a great example.