Tafsir of Yusuf 12:61

They said, "We will attempt to persuade his father regarding him," meaning we will deceive him, incline him toward our view gently, and exert our utmost effort in that matter. In this, there is an indication of the difficulty of the request and the arduousness of attaining it.

"And indeed, we will be doers," meaning we are capable of that and will not be hindered by it. We will certainly do it, without neglecting or delaying it.

According to the first interpretation, this sentence is a concluding confirmation that emphasizes the content of the first sentence and affirms the realization of what was promised, by using the effect (the act) to refer to the cause (the capability). According to the second interpretation, it is an affirmation of fulfilling the promise, and it contains nothing to indicate whether the promised outcome will be achieved or not.