Whale of an ace etc going amok (8)
I believe the answer is:
cetacean
'whale' is the definition.
(I've seen this before)
'an ace etc going amok' is the wordplay.
'going amok' indicates anagramming the letters.
'an'+'ace'+'etc'='anaceetc'
'anaceetc' anagrammed gives 'CETACEAN'.
'of' is the link.
(Other definitions for cetacean that I've seen before include "Animal order including whales and dolphins" , "One swimming" , "Eg, whale, dolphin, etc" , "Large marine mammal" , "Member of the animal order containing whales and dolphins" .)