Crossword Genius

Unfortunately, a gap his retirement left? (5)

Ross

I believe the answer is:

alack

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'unfortunately' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for alack that I've seen before include "unfortunately" , "Sadly" , "Alas (archaic)" , "Previously expressed regret" , "Old expression of dismay" .)

I've seen this clue in The Guardian.
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