Bus reversed over Laurel - extremely contemptible matter! (9)
I believe the answer is:
substance
'matter' is the definition.
(synonyms)
'bus reversed over laurel extremely contemptible' is the wordplay.
'reversed' says the letters should be written in reverse.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'laurel' becomes 'stan' (comedian Stan Laurel is an example).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'contemptible' with its middle removed is 'ce'.
'bus' back-to-front is 'sub'.
'sub'+'stan'+'ce'='SUBSTANCE'
(Other definitions for substance that I've seen before include "Physical material" , "wealth" , "Subject matter" , "Physical matter" , "power" .)