if i poisen a monster will they still drop a clue because ive been trying for a while and its not working

it shouldn't matter how you kill it, as long as it dies you will get the drops. Treasure Trails are very random, you can get 1 in 5 minutes of fighting, or 5 days of fighting.

The clue is a semi-rare kind of drop, don't expect to get them very often. It's also very dependant on luck, once I had 3 clues from guards within 1 hour of fighting and completing the previous clues, but for the next few days I got none.