Clue: McIlroy's first club covered constant tiny distance (6)
Answer: MICRON
Explanation: Definition: tiny distance
 **1st letter of**(…'s first) "McIlroy" + IRON(a club, as may be used by Rory McIlroy, professional golfer) **containing**(covered) C(mathematical symbol for a value that is constant).
Defn: … equal to one-millionth of a metre.