Flying ace describes Air Force in which one might serve (6)
I believe the answer is:
carafe
'one might serve' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'flying ace describes air force' is the wordplay.
'flying' is an anagram indicator (letters fly around into new positions).
'describes' means one lot of letters goes inside another (describe can mean to form the outline of).
'air force' becomes 'RAF' (Royal Air Force).
'ace' anagrammed gives 'cae'.
'cae' placed around 'raf' is 'CARAFE'.
'in which' is the link.
Can you help me to learn more?
(Other definitions for carafe that I've seen before include "Open-topped glass flask" , "Wine server" , "Open-topped glass container" , "keeper of wine" , "Wine, water, jug" .)