Clue: A jolly member (3)
Answer: ARM
Explanation: A jolly member (3)
A + R.M. (Royal Marine", hence "jolly") 
"Jollies" was the original name for citizen soldiers of the Trained Bands of London in 1664, from whose ranks Marines were first formed. Later, "jolly" became Royal Navy slang for a Marine, hence Kipling's 'HM Jollies'.