Encourage work on car that is inclusive of tax (8)
I believe the answer is:
motivate
'encourage' is the definition.
(thesaurus)
'work on car that is inclusive of tax' is the wordplay.
'work on car' becomes 'MOT' (MOT test).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'inclusive of' means one lot of letters goes inside another.
'tax' becomes 'VAT' (UK Value Added Tax).
'ie' placed around 'vat' is 'ivate'.
'mot'+'ivate'='MOTIVATE'
(Other definitions for motivate that I've seen before include "Give an incentive for action" , "Stimulate to action" , "Stimulate the interest of" , "Drive on" , "Prompt" .)