True song about source of caviar (8)
I believe the answer is:
sturgeon
'source of caviar' is the definition.
(I've seen this before)
'true song about' is the wordplay.
'about' indicates an anagram.
'true'+'song'='truesong'
'truesong' is an anagram of 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Fish valued for caviar" , "producer of expensive eggs" , "Fish producing caviar" , "Our gents, oddly, caught the fish" , "Caviare-producing fish" .)