When you say that someone is `crackerjack’ what you mean is that he is very industrious and very good at what he does. The word is considered slang.
Here’s an example.” His students say that Sanjay is a crackerjack teacher. But I heard that he is looking for a job that will pay him a better salary.” * ”I understand that Medini is a crackerjack batswoman. Her parents, however, don’t want her to take up cricket as a profession.” * ”I think Prem will make a crackerjack accountant.”