The most common and most likely answer to this crossword clue is the 5 letter word GLAIR
G
L
A
I
R
Buddy explains!

The straight part
We know that "Varnish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case varnish = glair.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 GLAIR mean?
GLAIR noun- Egg-white, especially as used in various industrial preparations.
- Any viscous, slimy substance.
- A broadsword fixed on a pike; a kind of halberd.
- To smear with egg-white.