Oil tax — one part adjustable — take it or leave it (8,5)
I believe the answer is:
optional extra
'take it or leave it' is the definition.
'oil tax one part adjustable' is the wordplay.
'adjustable' indicates an anagram.
'oil'+'tax'+'one'+'part'='oiltaxonepart'
'oiltaxonepart' with letters rearranged gives 'OPTIONAL EXTRA'.
(Other definitions for optional extra that I've seen before include "Non-standard feature" , "Non-obligatory addition" , "Non-essential additional item" , "not part of the package" , "It would be nice to have" .)