The most common and most likely answer to this crossword clue is the 5 plus 2 plus 4 letter word SCRUB UP WELL
I've seen this crossword clue in Financial Times
S
C
R
U
B
U
P
W
E
L
L
Buddy explains!
The straight part
We know that "become attractive when groomed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case become attractive when groomed = scrub up well.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.