The most common and most likely answer to this crossword clue is the 6 letter word WILDER
I've seen this crossword clue in Financial Times
W
I
L
D
E
R
Buddy explains!
The straight part
We know that "Film legend Billy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case film legend billy = wilder.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 WILDER mean?
WILDER verb- To bewilder, perplex
- Untamed; not domesticated; specifically, in an unbroken line of undomesticated animals (as opposed to feral, referring to undomesticated animals whose ancestors were domesticated).
- From or relating to wild creatures.
- Unrestrained or uninhibited.
- Raucous, unruly, or licentious.
- Visibly and overtly anxious; frantic.
- Disheveled, tangled, or untidy.
- Enthusiastic.
- Inaccurate.
- Exposed to the wind and sea; unsheltered.
- Hard to steer; said of a vessel.
- (of a knot) Not capable of being represented as a finite closed polygonal chain.
- Amazing, awesome, unbelievable.
- Able to stand in for others, e.g. a card in games, or a text character in computer pattern matching.