Clue: Using modified ten iron, his pitch changes little (7)
Answer: INTONER
Explanation: Definition: his pitch changes little
 **Anagram of**(Using modified) TEN IRON.
Defn: … when speaking or singing, say.
As to the surface, a golfer would use a pitching wedge, a club giving a higher loft than a 9 iron, used to make a pitch shot.