Estimating expenses of tonics mixed with gin, principally (7)
I believe the answer is:
'estimating expenses' is the definition.
I can't judge whether this definition defines the answer.
'tonics mixed with gin principally' is the wordplay.
'mixed with' indicates anagramming the letters.
'principally' indicates taking the first letters.
The first letter of 'gin' is 'g'.
'tonics' with its letters mixed up with 'g' can make 'COSTING'.
'of' acts as a link.
Can you help me to learn more?
(Another definition for costing that I've seen is " Selling for".)