I went off key in it and that's a pain! (6)

(Source: London Evening Standard)

"I went off" ⇒ anagram of "I went"

"key" ⇒ usually refers to a musical key ⇒ a letter from A to G ⇒ "g"

"in it" ⇒ one thing inserted into the other ⇒ "g" inserted into an anagram of "I went"

General meaning ⇒ a pain

Answer ⇒ twinge