Crude oil on tap? It's not necessary (8)
I believe the answer is:
optional
'not necessary' is the definition.
(I've seen this in another clue)
'crude oil on tap?' is the wordplay.
'crude' indicates anagramming the letters.
'oil'+'on'+'tap'='oilontap'
'oilontap' with letters rearranged gives 'OPTIONAL'.
'it's' is the link.
(Other definitions for optional that I've seen before include "Available as a choice but not obligatory" , "Platoon I may leave to choice" , "'Left to choice, not compulsory (8)'" , "By choice, elective" , "Elective, as you choose" .)