Free oil on tap is not compulsory (8)
I believe the answer is:
optional
'not compulsory' is the definition.
(I've seen this before)
'free oil on tap' is the wordplay.
'free' indicates an anagram.
'oil'+'on'+'tap'='oilontap'
'oilontap' anagrammed gives 'OPTIONAL'.
'is' acts as a link.
(Other definitions for optional that I've seen before include "'Left to choice, not compulsory (8)'" , "Platoon I may leave to choice" , "Platoon I join as a matter of choice" , "'Elective, not obligatory (8)'" , "Voluntary - not required" .)