Unfortunately, a gap his retirement left? (5)
I believe the answer is:
alack
'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" .)