Clue: Following Twain's lead, it's harsh describing golf as a long and tedious walk (6)
Answer: TRUDGE
Explanation: Following Twain's lead, it's harsh describing golf as a long and tedious walk (6)
*Following* **T**(wain's) [*lead*], *it's* **RUDE** ("harsh') *describing* **G**(golf, in the phonetic alphabet) 
Refers to Mark Twain's famous quote that "golf is a good walk spoiled".