Deep blue sea service (4)
I believe the answer is:
navy
This could be a double definition.
'deep blue' is the first definition.
(I've seen this in another clue)
'sea service' is the second definition.
'navy' can be an answer for 'service' (navy is a kind of service). I am unsure of the 'sea' bit.
Can you help me to learn more?
(Other definitions for navy that I've seen before include "Fleet - blue" , "Armed force" , "Dark shade of blue" , "Ships-of-war" , "military arm" .)