Sturgeon's playing a bugle (6)
I believe the answer is:
beluga
'sturgeon's' is the definition.
(beluga is a kind of sturgeon)
'playing a bugle' is the wordplay.
'playing' is an anagram indicator.
'a'+'bugle'='abugle'
'abugle' anagrammed gives 'BELUGA'.
(Other definitions for beluga that I've seen before include "Large sturgeon or it's caviare" , "Great Russian sturgeon - Arctic whale" , "Type of whale; sturgeon" , "fish" , "'Russian sturgeon, source of caviar (6)'" .)