One supporter upset about a canteen where you serve yourself? (5)
I believe the answer is:
naafi
'where you serve yourself?' is the definition.
The definition suggests a singular noun which matches the answer.
'one supporter upset about a canteen' is the wordplay.
'one supporter' becomes 'fan' (fan is a kind of supporter).
'upset' says the letters should be written in reverse.
'about' is an insertion indicator.
'canteen' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'fan' written backwards gives 'naf'.
'naf' placed around 'a' is 'naaf'.
'naaf'+'i'='NAAFI'
Can you help me to learn more?
(Other definitions for naafi that I've seen before include "Army canteen or shop" , "WW2 canteen" , "Military canteen" , "military institution" , "Forces 17" .)