Tax payer has SPIFF sales incentives to be reported on line 21. Can related deductible expenses such as mileage, telephone, etc also be claimed on line 21 as an off-set?
No - you will not be able to claim business expenses related to income being reported on line 21 as other "one-time" income.
However, if you report the 1099-MISC income as self-employment income on Schedule C, you would be able to claim any business related expenses.
Additionally, if you have unreimbursed job-related expenses that related to your entire employment (including the line 21 sales incentive income), then you will be able to include these as a miscellaneous itemized expense under Schedule A.
Please refer to this IRS link for more information about Business Expenses