Sentry spilled last of the oily liquid (7)
I believe the answer is:
styrene
'oily liquid' is the definition.
(oily chemical)
'sentry spilled last of the' is the wordplay.
'spilled' indicates an anagram.
'last of' suggests the final letters.
The last letter of 'the' is 'e'.
'sentry' with letters rearranged gives 'styren'.
'styren'+'e'='STYRENE'
(Other definitions for styrene that I've seen before include "Chemical" , "Petroleum by-product used to make plastics" , "Component of plastic" , "hydrocarbon" .)