How to use column alias to calculate other column value
Additionally to previous answers.

1) You can try to use user-defined variables:

SELECT 
    @earnings := (`house_rent`+`conveyance`+`medical`+`dearness`+`others_allowances`) AS earnings ,
    @deductions := (`income_tax`+`pro_tax`+`emp_state_insu`+`absence_fine`+`others_deductions`) AS deductions,
    @earnings - @deductions AS net_salary 
FROM 
    salary
    -- , (SELECT @earnings:=0, @deductions:=0) vars


Documentation do not guarantee the calculations order, but in practice all calculations are performed from left to right.

2) If your MySQL version is 8+, you can use CTE:

WITH cte AS (
SELECT 
    (`house_rent`+`conveyance`+`medical`+`dearness`+`others_allowances`) AS earnings, 
    (`income_tax`+`pro_tax`+`emp_state_insu`+`absence_fine`+`others_deductions`) AS deductions 
FROM 
    salary
)
SELECT earnings, deductions, (earnings - deductions) AS net_salary
FROM cte;