Good aquanaut's beginning to follow injured blue whale (6)
I believe the answer is:
beluga
'whale' is the definition.
(beluga is a kind of whale)
'good aquanaut's beginning to follow injured blue' is the wordplay.
'beginning to' says to take the initial letters.
'follow' means one lot of letters go next to another.
'injured' is an anagram indicator.
The initial letters of 'good aquanaut' is 'ga'.
'blue' with letters rearranged gives 'belu'.
'ga' after 'belu' is 'BELUGA'.
(Other definitions for beluga that I've seen before include "Great Russian sturgeon - Arctic whale" , "Species of sturgeon" , "Large Russian sturgeon" , "Large sturgeon, source of ..." , "Small white whale; type of caviar" .)