Ship's doctor eating last of succulent fish (8)
I believe the answer is:
'fish' is the definition.
(sturgeon is a kind of fish)
'ship's doctor eating last of succulent' is the wordplay.
'ship's doctor' becomes 'surgeon' (I've seen this before).
'eating' is an insertion indicator.
'last of' suggests the final letters.
The last letter of 'succulent' is 't'.
'surgeon' placed around 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "caviar suppliers" , "So urgent to find caviare-bearing fish" , "Something fishy here" , "Caviar supplier" , "swimmer" .)