An attack may be appropriate (3)
I believe the answer is:
fit
I believe this clue is a triple definition!
'an attack' is the first definition.
(I know that fit is a type of attack)
'may' acts as a link.
'be' is the second definition.
(fitting is a kind of being)
'appropriate' is the third definition.
(similar in meaning)
(Other definitions for fit that I've seen before include "Appropriate kind of convulsion" , "Sudden attack,of coughing say" , "Healthy; conform to" , "In good physical shape 93)" , "Certified OK by the doctor" .)