Cab mostly unoccupied is exempt from duty (3-4)
I believe the answer is:
tax-free
'exempt from duty' is the definition.
(I've seen this before)
'cab mostly unoccupied' is the wordplay.
'cab' becomes 'taxi' (taxi cab).
'mostly' means to remove the last letter.
'unoccupied' becomes 'free' (similar in meaning).
'taxi' with its final letter taken off is 'tax'.
'tax'+'free'='TAX-FREE'
'is' acts as a link.
(Other definitions for tax-free that I've seen before include "Exempt from duty" , "Exempt from a levy" , "Exempt from levies" , "Eg, VAT exempt" , "Exempt from tariffs" .)