Encourage tons to be rewritten about fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(I know that sturgeon is a type of ganoid)
'encourage tons to be rewritten about' is the wordplay.
'encourage' becomes 'urge' (I've seen this before).
'to be rewritten' indicates an anagram (I've seen 'being rewritten' mean this).
'about' means one lot of letters goes inside another.
'tons' is an anagram of 'ston'.
'urge' going within 'ston' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Fish providing isinglass" , "Caviare-producing fish" , "Fish sought for caviar" , "So urgent to find caviare-bearing fish" , "Source of caviare and isinglass" .)