Fish operator stores tonnes (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(type of fish)
'operator stores tonnes' is the wordplay.
'operator' becomes 'surgeon' (someone who operates).
'stores' means one lot of letters goes inside another.
'tonnes' becomes 't' (abbreviation).
'surgeon' going around 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Large fish, valued for its roe" , "Something fishy here" , "Fish from which caviar is obtained" , "So urgent to find caviare-bearing fish" , "Fish yielding caviar and isinglass" .)