What does the cryptic crossword clue "Communist officer seen on a merchant ship (3,6)" mean?

The wordplay uses a charade (building blocks). RED (synonym="RED") + ENSIGN (synonym="ENSIGN") = REDENSIGN The answer is REDENSIGN.

Communist officer seen on a merchant ship (3,6)

Guardian Cryptic #27359 | 9 Across | 2017-11-20 flower