Hi! I romanced Clarmont in my last run (took a couple runs). If I remember correctly, what I had to do was complete the Rebel secret/route, be an ethically moral MC, peace in Revaire AND use the PiratePad Week 5 Guide dialogue under the Angst Guide (Guide to not getting dumped). I believe you cannot be a Revaire widow. Note: I am not sure what the exact requirements are but this is what I'd done.
Amongst other things... Those were what I really had to focus on. Hope this helps!