Tons stored by operator, supplier of caviar (8)
I believe the answer is:
sturgeon
'supplier of caviar' is the definition.
'tons stored by operator' is the wordplay.
'tons' becomes 't'.
'stored by' means one lot of letters goes inside another.
'operator' becomes 'surgeon' (someone who operates).
't' put into 'surgeon' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Caviar-producing fish" , "Source of caviare and isinglass" , "Fish - got nurse (anag)" , "producer of expensive eggs" , "Fish that yields caviare" .)