A political clown and jester initially slipping over in oil (6)
I believe the answer is:
jojoba
'oil' is the definition.
(I have seen 'Cosmetic oil ' mean 'jojoba' so perhaps 'oil' could also mean 'jojoba')
'a political clown and jester initially slipping over' is the wordplay.
'political clown' becomes 'bojo' (nickname of Boris Johnson).
'and' means one lot of letters go next to another.
'initially' suggests taking the first letters.
'slipping over' shows that the letters should be reversed in order.
The first letter of 'jester' is 'j'.
'a'+'bojo'+'j'='abojoj'
'abojoj' back-to-front is 'JOJOBA'.
'in' acts as a link.
(Other definitions for jojoba that I've seen before include "North American shrub whose oil is used in cosmetics" , "desert plant" , "plant oil" , "Seedy desert native" , "tree" .)