Waste removal expert put gallons in brown car (4,6)
I believe the answer is:
dung beetle
'waste removal expert' is the definition.
'gallons in brown car' is the wordplay.
'gallons' becomes 'g'.
'in' means one lot of letters goes inside another.
'brown' becomes 'dun' (dun is a kind of brown).
'car' becomes 'beetle' (I've seen this before**).
'dun'+'beetle'='dunbeetle'
'g' inserted into 'dunbeetle' is 'DUNG-BEETLE'.
'put' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
(Another definition for dung beetle that I've seen is " Scarab, for instance".)