The most common and most likely answer to this crossword clue is the 8 letter word BRUSH-UP
I've seen this crossword clue in The Times
B
R
U
S
H
-
U
P
Buddy explains!
The straight part
We know that "Groom" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case groom = brush-up.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 BRUSH-UP mean?
BRUSH-UP noun- A slight correction or modification.
- A review of a topic to be learned.