First class aviators in the services (4)
I believe the answer is:
aces
'services' is the definition.
(ace is a type of serve in tennis)
'first class aviators' is the wordplay.
'first class' becomes 'ace'.
'aviators' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'ace'+'s'='ACES'
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for aces that I've seen before include "excellent service" , "Two will make deuce" , "Champions" , "wartime pilots?" , "Experts (in their field)" .)