Milton Terry, in Biblical Hermeneutics, proposes that in the history of biblical interpretation, Ezra (in Nehemiah 8.8) is the first explicit interpreter of the scriptures.
I'd suggest that the first interpreter is a bit earlier: Eve.
Eve (in Gen. 3.3) says to the serpent, "But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die," interpreting Gen. 2.17b, "Thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die."
I'm not certain whether Eve was right in her interpretation. She goes beyond what we know of the explicit word of God. But whether the interpretation is correct or not, she was explicating it. So the first honors goes to Eve.