Not the first mistake made by fielder? (6,4)

I believe the answer is:

second slip

'fielder?' is the definition.
(type of fielder in cricket)

'not the first mistake made by' is the wordplay.
'not the first' becomes 'second' (not the first but the second).
'mistake' becomes 'slip' (a slip is a blunder or mistake).
'made by' means one lot of letters go next to another (letters written or made by another set of letters).
'second'+'slip'='SECOND SLIP'

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.