What does the cryptic crossword clue "Submarine about to undergo a refit (1,4)" mean?

The definition is "Submarine". The wordplay uses an anagram. anagram of ABOUT = UBOAT; definition: "Submarine" The answer is UBOAT.

Submarine about to undergo a refit (1,4)

Daily Mail Cryptic #16843 | 12 Down | 2022-05-24 flower