The most common and most likely answer to this crossword clue is the 4 letter word BUFF
I've seen this crossword clue in The Times
B
U
F
F
Buddy explains!

The straight part
We know that "Polish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case polish = buff.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 BUFF mean?
BUFF noun- Undyed leather from the skin of buffalo or similar animals.
- A tool, often one covered with buff leather, used for polishing.
- A brownish yellow colour.
- A military coat made of buff leather.
- A person who is very interested in a particular subject; an enthusiast.
- An effect that makes a character or item stronger.
- Compressive coupler force that occurs during a slack bunched condition.
- The bare skin.
- The greyish viscid substance constituting the buffy coat.
- A substance used to dilute (street) drugs in order to increase profits.
- To polish and make shiny by rubbing.
- To make a character or an item stronger.
- To modify a medical chart, especially in a dishonest manner.
- Of the color of buff leather, a brownish yellow.
- Unusually muscular. (also buffed or buffed out)
- Physically attractive.
- A buffet; a blow.
- To strike.
- A buffalo, or the meat of a buffalo.