Attack decapitated blue whale first (7)
I believe the answer is:
seizure
'attack' is the definition.
('seizure' can be a synonym of 'attack')
'decapitated blue whale first' is the wordplay.
'decapitated' means to remove the first letter.
'blue' becomes 'azure' (azure is a kind of blue).
'whale' becomes 'sei' (Sei is a kind of whale).
'first' is a charade indicator (letters next to each other).
'azure' with its initial letter taken away is 'zure'.
'zure' put after 'sei' is 'SEIZURE'.
(Other definitions for seizure that I've seen before include "Sudden attack of illness" , "convulsion?" , "Paroxysm" , "Apprehension" , "Taking something" .)