Ensign's affirmative (3)
I believe the answer is:
aye
'ensign's affirmative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aye that I've seen before include "Affirmative answer" , "Word of assent" , "Positive voting response" , "An affirmative vote" , "Yes, at the double for the sailor" .)