What does the cryptic crossword clue "Aquatic bird I see leaving frozen mass, going north (5)" mean?

The definition is "north". The wordplay uses a reversal. EBERG (reversal="EBERG") = GREBE The answer is GREBE.

Aquatic bird I see leaving frozen mass, going north (5)

Guardian Cryptic #26527 | 8 Down | 2015-03-24 flower