30 weeks first pregnacy is it normal to leak a clear substance in one breast this early?

Breast discharge: At thirty weeks and sometimes earlier, secretions from breasts is most often just hormonal stimulation of normal tissue, preparatory to producing milk. Gently compress the other breast after a warm shower, and you will probably see a similar fluid from the nipple. You should consult your health care professional to be sure.

Not unusual: it is not unusual to have some nipple discharge during pregnancy. The only real concern would be if the fluid were bloody.