Not the senior service, but it goes far back (3)
I believe the answer is:
raf
'not the senior service but it' is the definition.
I can't judge whether this definition defines the answer.
'far back' is the wordplay.
'back' says the letters should be written backwards.
'far' written backwards gives 'RAF'.
'goes' acts as a link.
Can you help me to learn more?
(Other definitions for raf that I've seen before include "It fought Battle of Britain" , "Airforce of Great Britain" , "Royal Air Force (abbr.)" .)