Clue: Who in Paris contributes to reduced service in underground? (6)
Answer: MAQUIS
Explanation: Definition: underground
An envelope ('contributes to') of QUI ('who in Paris') in MAS[s] ('service') minus the last letter ('reduced'). Maquis originally was a Corsican word for dense scrub – "the bush" – into which someone who did not want to be found might disappear. It was applied to the French resistance, or underground, in the Seconf World War.