The most common and most likely answer to this crossword clue is the 6 plus 3 letter word BLOTTO (+)
I've seen this crossword clue in The Guardian
B
L
O
T
T
O
(
+
)
Buddy explains!

The straight part
We know that "Well-oiled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case well-oiled = blotto (+).The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BLOTTO (+) mean?
BLOTTO (+) noun- A person who is (very) drunk or intoxicated.
- An artwork created using blots of ink or paint.
- To become or cause to become (very) drunk or intoxicated.
- To be annihilated or destroyed; to be blotted out.
- (Very) drunk or intoxicated.
- A reagent made from non-fat dry milk, phosphate buffered saline, and sodium azide, which is used to block protein binding sites for laboratory techniques such as blots and the enzyme-linked immunosorbent assay (ELISA).
- On Sulawesi: a hollowed-out tree trunk used as a boat.