Clue: Finally serve ace, then service finding net (4)
Answer: EARN
Explanation: Definition: net
 **Last letter of**(Finally) "serve" + A(abbrev. for "ace", as in the playing card) **plus**(then) RN(abbrev. for the Royal Navy, one of the Services in the British military).
Defn: …/to gain as clear profit, after all deductions have been made.