The most common and most likely answer to this crossword clue is the 5 letter word ALCID
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The straight part
We know that "guillemot, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case guillemot, say = alcid.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
ALCID noun- A bird of the family Alcidae, including auks, auklets, razorbills, dovekies, guillemots, and puffins.