Rashi

Now a word was conveyed secretly to me He returns to his rebuke with which he has been chastising him till now: Why are you crying? “Remember now who was innocent that perished...As I have seen, etc.” And concerning you a word of prophecy was secretly conveyed to me, [lit. was stolen to me], from the expression like a stolen thing, because the Holy Spirit does not reveal itself to the prophets of the heathens in public. This is analogous to a king who had a wife and a mistress. When he would go to his wife, he would go in public, but to his mistress, he would go clandestinely and stealthily. So it is with the Holy One, blessed be He. Regarding the prophets of the heathens [Scripture states:] (Gen. 20:3), “And God came to Abimelech in the dream of the night,” and similarly (ibid. 31: 24), “And God came to Laban the Aramean in the dream of the night”; to Balaam (Num. 24:4), “who falls, but his eyes are open” [in Gen. Rabbah 52:5 (Num. 22:20), “And God came to Balaam at night”]; to Eliphaz, “in the visions of the night.” But concerning the prophets of Israel it is written (ibid. 12:8): “With him I speak mouth to mouth manifestly and not with riddles.” In Midrash Tanchuma, I found it emended [See Tanchuma Balak 8, Tanchuma Buber Vayishlach 24.]
Now [in] the expression, “And my ear grasped a bit of it,” שֶׁמֶץ, means a bit of the spirit of understanding.