The most common and most likely answer to this crossword clue is the 6 letter word DIGGER
I've seen this crossword clue in The Guardian
D
I
G
G
E
R
Buddy explains!

The straight part
We know that "Collier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case collier = digger.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 DIGGER mean?
DIGGER noun- A large piece of machinery that digs holes or trenches; an excavator.
- A tool for digging.
- A spade (playing card).
- One who digs.
- A gold miner, one who digs for gold.
- An informal nickname for a friend; used as a term of endearment.
- An Australian soldier.