A certain burden is on these vehicles (5)

I believe the answer is:

taxis

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'these vehicles' is the definition.
The definition suggests an adverb but the answer is not.

'a certain burden is on' is the wordplay.
'a certain burden' becomes 'tax' (I've seen this in another clue. I am not sure about the 'certain' bit.).
'on' means one lot of letters go next to another.
'tax'+'is'='TAXIS'

Can you help me to learn more?

(Other definitions for taxis that I've seen before include "return to right place" , "Cars with a driver for hire" , "Cabs" .)

I've seen this clue in the Evening Standard.

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