Stated opposition about very small service (4)
I believe the answer is:
navy
'service' is the definition.
(navy is a kind of service)
'stated opposition about very small' is the wordplay.
'stated opposition' becomes 'nay' (I can't justify this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'very small' becomes 'v' (abbreviated or small form of 'very').
'nay' going around 'v' is 'NAVY'.
Can you help me to learn more?
(Other definitions for navy that I've seen before include "Colour; fighting force" , "A dark blue; service" , "Armed force" , "Dark shade of blue" , "Blue - ships and seamen" .)