Solution – arable
When you look at the solution you can see that "arable" matches the meaning of "fit for ploughing".
It also matches the first part of the clue, in typical cryptic style ie "A" = a
"farmer finally"= last letter of farmer = r
Fit = able
A + r + able = arable
So you see, there was a reason for the word "fit" being in the clue twice!